Weber jetting question

[John Rogers]

I have a 2.2L race motor with large port "S" heads with some cleanup, 906 cams, 10:1 comp and it has the following in the Weber IDS carbs. 180 air corrector, F3 emulsion tubes, 160 main jets, 60 idle jets, tall secondary venturis and 36 mm primary venturis. The enrichment stacks were extended to go over the top of the tall secondary venturis. I checked the float levels before the last weekend's race and they are at the top of float gauge. Finally it has a set of headers from European George. The engine is dyno'd at 183 rear wheel HP at 6700 RPM. The problem is that during a race if I let RPMs drop below 4000 the transition back to full is terrible with a lot of stumbling and hesitation which makes me have to really concentrate on the tach and gears when I would rather be watching the other cars. The plugs are not black and neither is the exhaust. My A/F meter is nearly full scale rich at full throttle and slightly less rich at 4000 when accelerating.

Any ideas of what to look for? I have read most of the previous posts here on Webers.

[John Rogers]

Any ideas please?

[ValveFloat]

Ignore this post

[Grady Clay]

I assume you have 40IDA and OE intake manifolds (not 906), single plug, and close deck clearance (0.8 – 0.9 mm).

First you must realize those 906 cams are designed to only work well in the 6200 to 8000 RPM range. With careful work you can extend that to above 8000. They were also intended to be used with 46IDA carbs and the “tall” 906 intake manifolds. With everything working correctly and best possible high RPM HP, the engine will not work well at all below 4500 or so. It is unreasonable to expect even marginal performance full throttle at 4000 RPM. I find it works better at only part (20%) throttle at that low speed.

From your post, was your HP peak at only 6700 RPM? If so, there is a mis-match of parts here. You might get better performance with “S” or so cams.

Another important issue is gearing. On track with C6 cams and a small displacement engine (<3.0) having the exact right gears for the track is exceedingly important. You want to be able to shift at HP peak (say 8000) and have the RPMs be at torque peak in the next higher gear (say 7000). Most racers gear their transmissions for three or four gears around the track with top gear redline at the highest speed and second or third gear at torque peak in the slowest turn. Very track dependent.

Give us more details.

Best,
Grady

[Vintage911Racer]

That is the nature of the 906 cams. Anyhting below 4500RPM, the car will not run. An "S" Cam may be your next option.

You may also try rolling on the throttle as apposed to going full throttle right away. I know you want to mash it, but a slow smooth roll on the gas may help the problem.

[John Rogers]

Thanaks for the input guys. The tranny I used in Tijuana had K-Q-V gears for the top three and I never got into 5th except for a couple of times when I was alone and let the engine rest some. I also use 8mm wires, MSD 6A and Blaster coil and the plugs look perfect after a full bore run. I have tried rolling into the throttle but at times it is hard to remember that!! Grady, I had heard about the info you mentioned about the cams and the dyno runs were let off at 7200 RPM as power was dropping off some there probably due to the carb restrictions. The engine was built by Donnie Kravig's shop and was used for a race weekend before I bought it. The valve train is supposed to be good for 8500 RPM but I don't plan to rev it that high. Some of this is because I came from driving a stroker big four that had gobs of torque and didn't have to rev much to this new high reving six and I am getting used to it still? I'll take a couple of picts of the carbs and manifolds and post them shortly.

I did make some adjustments in the Tijuana race such as shifting into 3rd going around the traffic circle and keeping revs up to 5000 and I could shoot right out of the exit.

[John Rogers]

Here's picts of both carbs. I think these are the short or regular manifolds?

[Grady Clay]

John,

You have the OE intake manifolds.

Look at Bruce Anderson’s book, Second Edition, P. 88 for a good view of the
tall 906 intake manifolds. This is a book everyone should have.
"

"
(C) Porsche AG
(C) Bruce Anderson, 1996

Note the 46IDA carbs.
PMO and our host sells a current version, including manifolds.

Best,
Grady

[126coupe]

I agree w the above remarks its definitely the 906 CAMS

[jluetjen]

John;
I just spoke to Richard at PMO yesterday about carb parts since the engine that I'm starting to build will be very similar to yours. He specifically warned me about this condition. It would be even worse if you weren't using the tall secondaries.

My advice -- keep the engine between 5000 RPM and 7800 RPM. I wouldn't be expecting you to make peak torque until at least 5200 RPM. Your gears (K-Q-V) are not that long and only work out to a top speed of about 132 MPH with an 8000 RPM red line. It's curious that your HP was dropping off already at 7200 RPM since at that speed your 36 mm venturi had a gas speed of about 90 m/s which is about the same gas speed as the 68S was pulling at 6600 RPM with 32 mm venturi. I've seen people report on this BBS (as well as BA in his book) with configurations that were pulling upwards to 119 m/s at the peak HP engine speed.

BTW, 185 at 6700 RPM works out to about 210-215. Not too shabby for a 2.2. Imagine what it will do if you keep it "on-cam"! You've got a good 1000 RPM that you're not using!

[Frostie]

John, just curios, you mention that your running near full rich as indicated by you’re A/F gauge. Why don’t you lean her out a little? Just curios.

[jluetjen]

Another thought -- do you know (or can you get) what the cams were set too? The cam timing can have an affect on your rev range too. I doubt that it will help the 4000 RPM stumble, but it may help you find another 500 RPM in your rev range.

BTW; Grady, do you know or remember what the venturi size was in the Weber 46's?

[John Rogers]

Thanks again guys. I had figured it is the cams and if I can get used to revving the engine I'll be okay. I think the full rich comes in from the "candy canes" finally kicking in at 7000+ RPMs. When we first started with these carbs, they were bent off to the side and did not go over the top of the tall secondary venturis. I added some extension pieces of copper tubing to make them hook over the top of the venturis and that is when the extra richness sets in. The exhaust is not black, actually really clean and the plugs are just about perfect.

[TimT]

OK... thought id add my 2 pence

cam timing isnt going cause stumbling You can move your power up and down via cam timing..

fuel delivery, ignition advance, and cam timing are all related variables on this engine..If you have balls out power ant certain rpm ranges, you have the proper magic combo...

how is the fuel delivered? robust pump? have the float bowls been modified?

I think you just need fuel

Of course many of you will say Im wrong

[John Rogers]

Good point Tim, at the time of the dyno testing I had the same fuel pump used with the 2.4l four and have since gone to a larger capacity pump (25 GPH at 4.5 PSI). I think this is much better. Another thing I am wondering about are the air cleaners as you can see from the picts that are not very tall and I am concerned they are restricting flow at very high RPMs. I may try a run or two at the upcoming CA Speedway event to see if they are a factor but I don't want to suck in too much dust and dirt.

[jluetjen]

As someone else once observed, "if you've eliminated all of the obvious suspects, then the Butler must have done it!"

If you can isolate that it is not the fuel pump nor the air cleaners which are limiting your rev's, you might want to confirm the ID of your headers. Having them be too larger or too small could affect the running of the car. If you've eliminated all of those "simple" bolt-on items, then you may be stuck with the combination of venturi size and cams. Either the venturi size (and carbs since I doubt tht you can go much bigger then 36 mm in a 40 mm carb) is too small or you've got too much cam. You may find that you can make almost as much HP and have a much more flexible engine by switching to something like an S cam.

[Grady Clay]

John,

I can’t find any original C6 carb specs. I know I had them many years ago. I’ll keep looking.

For my GT2 application, the venturi is 42 mm, 175 main, 145 air on a 66x91 cylinder (2576 cc).

This was last run in a 3-hour Regional enduro on a small track and at 5400 ft altitude. Needless to say we were working hard to get good fuel economy. I added a second small fuel cell as a two gallon surge cell and a recirculating fuel pump so I could run the main cell dry and then some. It had a trailer light and fuel pressure switch on the recirculating circuit to tell when the main fuel cell was out of fuel.

Best,
Grady