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Keihin Carb Conversion

11/24/01 -- This one is strictly in the "fun and games" department. I have a perfectly good, functional Weber 32/36 DGV downdraft setup thanks to the PO and really can't complain about it. Decent performance, great reliability.

But just the thought of having one carb per cylinder just lights me up. There are plenty of technical reasons that a better breathing carb setup is worth pursuing -- very few that I can really speak about with any kind of intelligence or authority -- so I won't. But suffice to say, plenty of guys who are a lot smarter than I am when it comes to this stuff think that this sort of conversion is the cat's meow.

One company that builds racing engines sells these things in kit form for about $1,149 or something to that effect. It's a do-it-yourself kit that contains everything you need and the carb is pre-jetted for the engine it's being installed on. Mikuni also makes motorcycle carbs that will work, but from what I've read, the Keihin's are much easier to tune and keep in tune than the Mikuni's or Weber DCOE's, which this setup will more or less emulate for a fraction of the cost. These are the actual carbs that I installed. You want to find Keihin CR Specials. They use a round slide technology and operate much in the same fashion as those trusty SU's, except they're purportedly a lot more efficient and easier to keep in tune.

When I saw this set of carbs on eBay for what ended up being $300, I couldn't resist the notion of trying to assemble a performance carb setup myself and bypass the middleman. I spent another $150 on air filters and a few miscellaneous parts that were missing from the carbs, though they appeared to be in nearly brand new condition as you can see. The carbs that have been successfully converted for use in Spitfires so far so far feature 31mm to 33mm bores for 1500 engines. The carbs I wound up with are 29mm, which I'm told might work out even better for my engine and provide better low RPM performance. I'll find out soon enough...I would think that this size would be better suited to the earlier 1149 engines on the original Spitfires, too.

The real trick has been in the fabrication of the custom manifolds that will be necessary to facilitate the installation. A member of the Totally Triumph Network who asked to remain behind the scenes offered to machine me a pair very similar to the ones he has in mind for his fuel injection project. They're machined from billet aluminum and will connect to the carb intake tubes via steel reinforced high temperature rubber hoses.

I've been waiting for a few months to receive the manifolds, as the gentleman fabricating them is even busier than I am, but I'm told they'll be arriving this week sometime. In the meantime, I've kept myself busy gathering the rest of what I'll need to do the conversion, including a cable operated throttle and a welder that I'll use to fashion some sort of support bracket. Once the manifolds arrive, I'll kick this project to the front of the pile and see if I can make it all work. It should prove to be very interesting...

1/13/02 -- The aluminum billet manifolds took a little longer to get here than expected, but turned out to be a nice present just a few days before Christmas. The gent who created them for me also provided heat shields, two kinds of hose sets, clamps, and all necessary hardware to install the new manifolds. This was much more than I was expecting and I'm very grateful for his time, efforts and help. As you can see, they are "handed". The intake tubes (which are nicely barbed, you just can't see it from this photo) on the outside are angled out slightly. The inner tubes are straight because there is a straight path to the Keihin's when they are fitted to the manifold. In hindsight, slightly longer tubes would have been preferable for clearance purposes, but these still did the trick nicely. How much longer? 1/2 inch to an inch max would be terrific.

I accomplished quite a bit in a few hours today, but there's quite a ways to go. The first thing I had to do of course, was remove the Weber DGV. I did this with a semi-heavy heart, because the Weber has been working very well and I have no complaints at all. Now, I'm venturing into uncharted territory. Well, I do have one complaint. The design of the Pierce manifold, which I've never had any problems with until now, prevents easy access to the lower clamp nuts, requiring a very awkward approach with an open-end 9/16" wrench that only has enough room to loosen the nuts one flat at a time. Getting these loose enough to remove the DGV manifold proved to be a real lesson in patience and frustration. Talk about grunt work! But eventually, I prevailed, connecting the hoses that passed through the Pierce manifold together temporarily with a two-way PVC fitting.

As you can see, the stock cast iron exhaust manifold has three mounting holes to support the stock intake manifold. These mounts will be used to secure the Kehin's eventually. 'm going to weld up a narrow H-shaped bracket. Picture the H lying on it's side and look at the photo. One side of the H will be bolted to those three mounts. The other long side of the H will support the carb set.

A real handy set of tools that were included were comprised of two PVC pipes which were inserted into the manifolds and pressed into the ports on the head to insure a perfectly aligned fit.

Knurled allen headed bolts were used to replace the stock studs on the cylinder head for mounting the new manifolds. The lower clamp studs and nuts were retained since tightening them with a long socket would be easier than getting an allen wrench under there. This didn't seem to pose any problems. The following shot shows both manifolds installed.

At this stage, it didn't take long to get the Keihin's mounted up, but I discovered that the hoses are 1"-2" too long, so they'll need to be cut down. But we're getting there...

I rigged up a very temporary support using a discarded part from an old telescope, which is attached to the center manifold mount and simpy extends to the underside of the Keihin's. I'll need to whip up a proper mount before I take the car for a drive. Hey, that's sounding optimistic!

Next up, connecting the fuel line and the throttle linkage. I'm about a foot short on the fuel line, so that'll be my next purcase, along with a return spring for the carb throttle. At that point, it'll be time to secure the carbs and crank that starter key to see what happens. I'll have my fingers crossed...

Later that evening... Once I removed the flexible thinner tubing in anticipation of cutting it down, I decided to give the thicker tubes a go, just for fun. I got them to fit, but it's pretty tight on the outside edges where the rubber wants to come in contact with the exhaust manifold.

Again, hindsight (always 20/20) would seem to dictate that we probably would have been better off with slightly longer tubes on the intake manifolds. Another inch or so would make the hose attachment a lot easier, I think. I should have known better when I had trouble fitting the thick rubber tubes to the carb mount adapters. I actually needed a little WD40 and had to rock the rubber tubes back and forth until they fit the adapters all the way down. I was hoping that since I was able to get them to seat fully, I wouldn't have to cut them down. No such luck. They were still too long, but like we discussed, better to have to trim than fall short.

I decided to cut down the thin hoses and use them again, since it should prove easier to install the heat shields with the thinner walled hoses.

Oops! No amount of rocking, twisting, oiling, etc, was going to get the thick hoses off the carb mount adapters. Any more force and I was afraid I'd bust something. Fortunately, the carb adapters are threaded, so I simply unscrewed them from the carb body, then used my cutting wheel to nip an inch off the opposite end of the thick hoses. I dragged the cutting wheel out to my driveway and donned my two stage respirator and face shield before cutting. It was messy and smelly, but the cutting wheel and the wire cutter seemed to do the trick nicely. I should have only cut maybe 3/4" off the hoses that attach to the angled manifold tubes, but I was able to still get them on far enough to get a good bite with the hose clamps. And yes, I made certain to THOROUGHLY clean out the rubber debris from each hose before reinstalling.

There's no way that I'll get the thick hoses off the mount adapters now -- at least not without cutting the hoses (and that's far from being an easy proposition), but at least the carb adapters can be replaced at a later time if need be. My only challenge/struggle now will be in fitting the heat shields.

I didn't get a chance to get the fuel line today, but managed to rig up a strong support for the carbs. All that's left now is to attach the fuel line and see if I can get the thing to run... I've decided to hold off on installing the cable-type throttle until I determine if these carbs are going to work. Once I get them running, swapping the mechanical gas pedal for the cable type should be the easiest component of this project. The real trick is going to be rigging the choke cable. Right now, the choke on the carbs is activate by a little lever, that's meant to be actuated by hand. I'll have to figure something out...

1/15/02 -- Today was the day! Almost. Well, we have ignition at least! I still need to connect the cable throttle and install the heat shields somehow, but I managed to start the car up today and it ran smooth with an occasional backfire out of one of the carbs. I'm certain that it's running too richly, since no choke was needed at all to start the engine from cold (and for San Diego, it's a cold day at 60 degrees and even cooler in my garage)

I tried leaning out the mixture, but the needles that are already in there are still too rich. I discovered a small drip out of the forward carb closest to the fuel inlet which is in the center. I'm hoping this is merely an indication of the rich running mixture -- that the fuel can't be used quickly enough by the engine and the excess is pooling from the bowl of that particular carb. At least, I sure hope that's all it is... It seemed to stop on it's own after awhile, but I'm keeping my eye on it.

But I'm very heartened so far... :-) I didn't smell any rubber melting or noted any problem with the hoses that are closest to the exhaust manifold... I can't wait to get everything buttoned down and go for a test drive soon.

1/18/02 -- I was up last night and past Midnight working on the final stages of this conversion. But before that, I trimmed about an inch off those thick steel reinforced Kevlar air hoses, bringing the total distance from cylinder head to the tip of the velocity stacks on the carbs to 12". I may still trim another inch off, though I have no problems with the inner wheel arch clearance when opening and lowering the bonnet.

The main difficulty yesterday had to do with the gas pedal. I had acquired two different cable-type pedals, but both are naturally LHD. I didn't realize that these were "handed", but they are and the one I used required significant bending to work properly for me. Even so, more adjustments are needed. The pedal sits too close to the brake and about an inch closer than the brake and clutch pedals. I connected a return spring to the carb so that it returns to idle, and it was a strong spring too -- but not quite strong enough for the kind of quick response I'm after. I'm going to add a second spring shortly.

I feel embarrassed to admit this, but before last evening, I was having a real difficult time determining how the throttle cable sheath on the pedal side is supposed to be held in place. The answer was a lot simpler than I was imagining. Just a simple, 5/16th hole drilled through the tob bulkhead shelf directly above where the pedal extension ends, right above where the cable inserts and is clipped to the pedal shaft. After press fitting the heat shields into place temporarily, and making sure there were no leaks, I cautiously backed Mrs. J out of the garage and took a short test drive around the neighborhood -- I didn't want to go too far in case I had a problem.

I could immediately tell a difference in power over my Weber DGV and the whole engine sounds more... authoritative for lack of a better description. I came back home after my quick little jaunt and double checked everything under the hood, then took a better test drive on the main boulevard near me. To sum it up -- it feels like I have a whole new car.

I read something that Chris at PRI race wrote about these carbs and didn't really quite understand what he meant when he said you have to get used to "rolling into the pedal" rather than just mashing it down. My feeling is that when I'm at full throttle, too much fuel is bogging things down or the stock manifold is too restrictive. As long as I'm easy on the pedal, I can fairly quickly and smoothly get to redline, which in my case, I keep around 5,000-6,000 RPM. While at cruising speed, a light tap on the pedal produces a noticeably stronger surge than I've experienced before, lightly pressing me into the seat back.

Another thing I noticed is that when decelerating down a hill for instance, I dont get any more of the raspy phhh-ut-phpput sounds like I was getting from the Weber that I could never figure out how to get rid of.

I'll be interested to hear opinions on the low to mid range bog down on full throttle. There is no question now that headers, and eventually, a lightened or perhaps even an aluminum flywheel will be in my future. When I was starting at a stoplight, I decided to try and move through the gears as rapidly as I could since there were no officers of the law present at the moment, and for the first time ever, accidentally squeeked the rear tires a bit in first. The surprising part for me is that it occurred after I'd already started rolling. The rest of the traffic was far behind me in my rearview mirror about 1/4 mile down the road when I had to slow down to make the turn into my neighborhood.

I DO need to make a critical adjustment, however. While I had to bend the pedal to fit my RHD application, I may have bent it slightly too far to the left (it's closer to the brake pedal than I'd like) and I also need to bend it back some, since it also is further forward than either the brake or the clutch, which feels real odd. But after I do that, it looks like this project is complete after I replace the velocity stacks with air filters for regular road driving!

It'll be interesting to see what sort of mileage I get from these things and also to check the plugs after I've driven several more miles. I'd like to learn how to lean out the carbs in the low-mid to mid RPM range if possible, because then it would be a really smooth without my having to consciously keep my foot off the floor, ha ha. BTW, I saw no evidence of low power in the higher RPM ranges whatsoever. Seems to do really well there...

1/21/02 -- More fun and games since my last entry. These carbs have adjustable needles via grooves cut into the top of the needles and a small C clip that is placed in one of the grooves. Raising the clip position physically lowers the needle, leaning out the mixture. Lowering the clip position raises the needle, richening the mixture. Since I'm running too rich, I raised the clip on each carb needle by two or three slots (and I have two more to go), turned the mixture screws all the way to bottom and opened a half turn. Yesterday, I took the car out for a drive and while I noticed an improvement in throttle response to some degree, the pervasive smell of gasoline was my traveling companion. -- not good.

I attributed this to a still too-rich setting. Upon entering my garage last evening, I was alarmed to discover a pool of gasoline beneath the car and a steady drip coming from the carbs. I had no idea that the gas could leach out of the tank like this with the fuel pump off, but there must be some sort of siphoning effect at work. I removed all the air cleaners and started the car up. Fuel was spurting out of a weep hole in the #1 carb. A LOT. When I turned the engine off again, I could see the fuel cascading down the throat of the carb. After blocking the fuel line, I removed the carbs today and removed three of the four float bowl covers, including the one causing me trouble and closely inspected each of them, looking for any differences, since the first one didn't seem to be binding and appeared to be in good shape. There was no corrosion or gunk of any kind. They were real clean. I seemed to detect perhaps the slightest difference in the tang that presses against the valve to close it on the leaking carb, so I bent it down a bit flatter to match the others. By doing this, it would engage the valve that much quicker as the float level rises.

This strategy seems to have paid off. I reinstalled the carbs connected the fuel line and started the engine up. No more leak. The engine ran really well, too. I let it run for quite a while in my garage with the air cleaners off, then checked again for leaking out of the weep hole or the throat of the carb. Nope. Dry as a bone. I drove the car a little while later and the thrilling scent of gas leaking onto my manifolds is no longer in the air (actually, it was never dripping onto the manifold itself, I don't think.) I still have that bodgy mid-range performance, but I'll get to the bottom of that shortly. The top end performance is very, very good. Makes me want to drive around with my engine screaming at 4000-6000 all the time...

To be continued...

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