Weber hard acceleration problem andy advice

[haasad]

Hi, I used my car in a competitive hillclimb this weekend for the first time this weekend . I noticed that under hard acceleration rather than a more progressive throttle application I would usually use the motor stutters before cleaning up and pulling. The stutter is only probably a second or so but I really lost time.

Never noticed this on the road. Its a 2.8 twin spark 6 deg BTDC idle...25 degrees all in at 3500rpm. 10.5:1 comp S cams webers with 36mm chokes F3's, 180 airs 150 mains. Carb balance etc all good tickover clean but maybe a bit rich. Shafts rebushed new butterflys etc

I'm wondering if the 36 chokes are too big since its got the heads from a standard 2.7 CIS car from 75 which I think had smaller inlets.

It feels like a two stroke mx bike that's gassed up not responding to the throttle then clears if you know what I mean .

Any ideas or advice where to start looking

[RSTarga]

Is your float level set properly?

[1QuickS]

Accelerator squirt amount may be a little weak. Also, there are three types of cam/levers that actuate the pump. The earliest ones have very short arm and use a fixed length rod from the throttle shaft. The next version has same cam profile as the earlier version but with a longer arm to accommodate the adjustable pump rod feature. This cam/lever actually has a slower squirt response than the earlier version due to the shorter lever arm. The last version had the longer lever arm but a steeper ramp, this type was used on 1969 911T and all 914/6 Webers. The cam/lever looks like a hatchet. This last version would help provide a more aggressive squirt when throttles are rapidly opened. All three have same total lift.

Jetting sounds about right. However, if you are using IDTP throttle bodies then you might be suffering a leaning of the progression which hurts mixture at lower RPM also. If you think this might contribute then there is a way to tailor the mixture profile across progression by adjusting idle air bleed jet size.

[haasad]

Thanks both . floats are bang on ( but will check again) fuel pressure steady 3 1\2 psi. I have the early fixed accel pump link and cam. The links are in good condition but I'd be happy to change to a later better set up if I knew where to get the parts.

I assume the carb numbers on the body will identify the exact model ,I will check and get back.

Seems you both think this is a lean condition or issue? Any quick ways to prove ?


Thanks for the help so far.

[1QuickS]

Yes, lean. Sudden demand for air and fuel from mains lags which is what accelerator squirt is for, to bridge the condition until fuel delivery from the mains catches up to air flow.

I worked on a set of 46 PMOs with a similar issue, I put them on my 2.3 twin plug, S-cammed motor. I used 36mm main venturis for testing. Huge lean hole making them seem like they were falling on their face, almost undriveable. After adjusting the squirt volume up, installing the "hatchet" cam-levers and making the idle air bleeds smaller I achieved a reasonable result for transition performance. Subsequently, PMO replaced their accelerator cam/levers with the "hatchet" style.

You have a 2.8 with 36mm main venturis in 40mm throttle bodies and I have 35mm main venturis on my 2.3 with 43mm throttle bodies and they work very well, thank you! Your venturis and throttle bodies are fine for your engine and for throttle response.

Difficult to test since this is a acceleration issue and not a steady-state or moderate acceleration operation. Since you seem to be satisfied with normal operation and progression but with sudden throttle opening you get a stumble then attention to acceleration richness is in order. Lean will respond like the engine quits or stutters, rich will have a sluggish response. Try this: Open throttles until situation occurs and then just let the throttles close by 10% or so; if engine responds favorably then this indicates lean. The slight throttle closing allows fuel to be more correct strength to a reduced air flow. Remember that air flow responds VERY quickly but due to the density of the fuel mixture it cannot respond as quickly as does the air. MFI does not suffer this lag, carbs do demand a bit more driver sensitivity to their limitations.

You have early cam/levers which have a fairly quick fuel response, I believe the "hatchet" cam/levers were developed after the long arm cam levers were found to be less aggressive than the earlier versions. I have modified early cam levers to use adjustable pump rods. You cannot increase squirt amount with fixed length pump rods but you CAN decrease squirt by bending the links to make them effectively shorter. Possibly you could make some temporary links that are longer. At least it is important to KNOW what the fuel injection amount is and that all six squirts are close to being equal. Typically .7ml/squirt/nozzle is expected but that was the spec for the 2.0S engines, you have a 2.8... It would be good to perform a squirt volume measurement to know where that stands.

[Steve@Rennsport]

Excellent advice & suggestions from Paul as usual.

Q: Are you running single-ignition or twin-ignition?

25 degrees total is fine for a twin-plug motor, but pretty light for a single-ignition one.

Hopefully, you are not compensating for that CR on pump gas.

[haasad]

Thanks all, I haven't had a chance to check exact carb model but will. during tomorrow afternoon. Pump gas, yes, but twin plug hence the light timing.

Rolling the throttle doesn't give the problem but I will try the "back off" test too.

I will also measure accelerator pump delivery in each venturi.

I'm confident I could make the hatchet type cams if I had to but will ask at PMO to see if their part fits. Do I need a particular operating rod to go with that type cam I think I understand the longer rod is neccesary, if so is it likely PMO will have them too?

Thanks so much for the help with this.

Andy

I will report back

[chris_seven]

Andy,

Do you know what size pump jets you have fitted and also what size of pump discharge valve is installed?

If the delivery volume if the pump needs to be increased then a larger pump jet may be needed and then it could also be important to reduce the size of the pump discharge valve to obtain the desired output.

[1QuickS]

PMO "hatchets" are thicker than OEM cam-levers. They provide spacer washers for the fulcrum screw to correct for this. They also offer adjustable pump rods which come with the boss that attaches to the cam-lever.

If you do not get the .7ml/nozzle as Chris_Seven mentions then your inlet valve may be bypassing gas, either through a bleed-off hole in the side of the valve body (not a common version of the check valve) or by poor operation of the check valve itself. Check for the latter by removing top of carb body and operating throttles. If valve is closing properly then you will not see a disturbance in the fuel in that bowl, a leaky check valve will swirl the fuel above the valve indicating fuel is bypassing the valve.

Nozzle size affects how strong the squirt is but squirt volume is not affected. Larger hole in nozzle decreases stream intensity and shortens duration of squirt and conversely.

While checking squirt amount you will also want to check to be sure the squirt does not squirt onto the wall of the main venturi. The squirt wants to hit the throttle plate. There are a few different squirter nozzles that have either different lengths or different angle of discharge. You should be good with any since you have large inside diameter main venturis.

[haasad]

Thanks both, the accelerator nozzles are delivering just on 5 m/l per choke and I mean just. the rh carb is slightly better than the lh.

The jets are pointing square down the venturi and onto the throttle plate. I need to check the accelerator pump inlet valve for leaks & jet size don't I ? That seems too little fuel and is maybe the first port of call before I change the rods and accelerator pump cams .

Does that seem sensible ?

[1QuickS]

Squirt amount needs to be more, for sure.

[haasad]

I have spent some time checking the pump circuit. The ball valve in the lhs carb was actually loose and passing, so that explains the mismatch side to side.

Its clear that the pump stroke delivers a finite amount of fuel. I looked at the accel jets each one was marked 50 and 3 degrees; presumably 0.5 mm hole injected at 3 degrees to vertical. l decided that I would open these to 0.7 mm diameter from their original 0.5 mm.

The stutter is definitely better and the pick up crisper but still there, any thoughts on increasing the jet size further or is it time to swap to the hatchet cams and rods...

OR...

another demon tweek since the evidence is we are on the right path ?

Thanks again I'm enjoying this

[1QuickS]

Yes, 50 = 0.50mm orifice diameter and 3 is the angle of squirt from vertical. There are squirters with a 2 degree angle and those with a long nozzle that extend nearly to the outside diameter of the auxiliary venturi as used on the 2.0 911T of 1969 and the 914/6 engines. The 3 degree is good for you. Enlarging the holes will speed up fuel delivery. Adjustable length pump rods and the hatchet cam lever are next.

Increasing volume out of the pump is not so easy.

When I service a set of Webers I disassemble the inlet check valves, clean the seats and install new check balls, they work well when this is performed!

[haasad]

Brilliant , I should have said I did the check valve test looking at the fuel in the chamber, that's how I identified the one loose and passing. I also took them out to check seating they held air and brake cleaner so I,m confident in their seating.

Is it possible I could get the main dimensions of the hatchet cam and adjustable rod even by pm since they are very easy to make, or can I buy them from you at Performance Oriented seems better to do that given the free advice I am getting? I am in the UK though ?

[1QuickS]

Look at your cam levers, you can use your early version and continue the arc of the cam and continue the straight portion of the lifting ramp and let them intersect. This will create a hatchet cam profile. The short lever arm is fine for now. If you wanted to get adjustable link and new hatchet then please contact me via my email address:

info@PerformanceOriented.com



Picture of adjustable pump rod with the boss that connects to the cam lever, not so easy to fab all this and to have it clear the short lever arm on your cam lever.

Note that washer on boss would interfere with roller/follower on your arm that is located in the top cover of the accelerator pump if you used the early cam lever with the short lever arm. You can see the smaller hole for the pump rod in the earlier cam lever (Posting #3); later cam levers have larger hole to receive the boss.


Glad to be of service, I learn by helping others.

[1QuickS]

Another few items to consider to improve output:

1. Increase spring pressure of discharge spring in main pump body
2. Make new pump rod link
3. Make new cam lever with increased lift.

Suggestion 1:
The discharge spring provides the pressure for the squirt, a stiffer spring or one with more pre-load will make the squirt both stronger and shorter in duration. You can increase spring stiffness/preload by:

• put a spacer under the bottom of the spring to increase pre-load, being careful to keep spacer thin enough to not have the spring go "solid" during installation (most springs have very close coil spacing so shimming is not a good path)
• Physically pull on the spring to elongate it, this will increase pre-load
• cut some coils from the spring and elongate it. Cutting coils from the spring effectively increases spring-rate
• source a replacement spring.

Suggestion 2:
Make a new link of fixed length like the OEM one but be sure the new link locates the roller/follower in the crook of the cam-lever. The OEM link may be worn which will decrease the effective length (decreasing squirt) and the OEM link may not establish the roller/follower at its best position.

Suggestion 3:
There is more available fuel within the outer chamber of the accelerator pump for discharge that is not accessible due to lack of total lift of the cam-lever. You can check for how much extra lift a custom cam lever can have by opening throttle to WOT position and then use your fingers to actuate the arm in the top cover of the accelerator top cover. You will see what fuel is available (more squirt after OEM cam has maxed out) and will also be able to determine how much extra lift a custom cam-lever can have.

[chris_seven]

I still think that the size of the discharge valve can affect delivery and it would be interesting to know what size is installed.

The discharge valve will affect the amount of fuel being returned to the reservoir.

A size 100 gives the maximum return flow and a size 0 gives no return.

If you make a new cam from a steel plate you will need to case harden the steel or it will wear quite quickly and the steeper the ramp the faster it will wear.

[1QuickS]

Right, inlet check valves can be drilled with bleed holes, the early 40IDA3C Webers like Hassad has did not use them but they could have been retrofitted. The bleeds were a method for adjusting squirt output (decreasing it) prior to the development of the adjustable pump rods which allowed adjustment of squirt without disassembly of the carb.

Hardening new cam levers is good but life is probably not a huge driver since mileage of these cars probably won't be that great compared to when they were new. Besides, making a pair of "test" cam levers would prove the theory and after finalization a proper cam lever set could be made and heat treated.

[chris_seven]

Interesting 46IDAds used a 50 pump jet and a 40 discharge valve. I am not sure about the cam shape but I will look.

I have a number of 'hatchet' style blanks I have just sent for hardening.

[haasad]

Wow, great discussion and knowledge.

Paul I noticed the extra throw that is available on the pump arm and can see how a cam with more "lift" could utilise that.

I'm less comfortable at this stage with the adjustments to springs that are suggested since I hope to get what I need with the changes you have given me help with so far and perhaps some different cams albeit with those optimised in terms of rod linkage and indeed timing.

Chris am I able to buy a pair of the "hatchet " cams from you ? saves me making them. Yours may last longer too

I need to clarify pump jet and discharge valve.

I didn't note a number on the ball valves ( pump jets ?) in the float chambers. They definitely don't have any drillings though, so I assume they will deliver within the limits of the main hole diameter whatever the pump sucks ?

The discharge valves are I assume the the little drilled arms that project over the venturi ? Mine were 50 (0.5mm) and are now 70 (0.7mm)

Am I missing something here ?

Andy ( AKA Mr very grateful for the advice)