Fuel Starvation?

[Petrolhead]

1330 engine producing abiout 95 BHP with pair of HS4 SU Carbs, 286 cam, The cylinder head is a Swiftune 37in 29ex, needles are AAB. Going for rolling road this Friday, hence why I would like to get this sorted before going.

Dizzy was modified as per Keith Calver recommendation, he supplied the head and cam. Total timing is 28deg

When giving the car welly once it gets to about 5500rpm the car feels like its got fuel starvation ie it just stops accelerating and I have to back off

Its currently running a SU pump mounted to the sub frame

So is this a likely cause and if so whats the best solution?

[Kdb550]

If you back of ever so slightly does it make a difference or do you have to back off all the way, I have had this before when the carb has not been set correctly and runs to lean on full throttle. just allowing the needle to drop slightly brings it back in. If you take the plugs out how do they look do they look like they have been getting to hot. If its going for rolling road setup I would not worry yourself to much as they can probably sort this out anyway. Could also be the dizzy is advancing up to much at higher revs.

[Petrolhead]

Difficult to answer as not realy analysed what I have done when this happens but RR session tomorrow.


Dizzy is giving 28deg max

[FWDRacer]

AFR reading based on the exhaust output will be the key here. Sounds like a high rpm lean out - Be careful on the rollers and tell the operator before he runs the car-up.

As the poster above has stated - does the car accelerate (surge) when you back off from WOT at high rpm?

Edited by FWDRacer on Friday 10th May 08:49

[guru_1071]

28 degrees doesn't seem enough for a tuned 1275, I normally aim for 32-34 total

[Petrolhead]

guru_1071 said:

28 degrees doesn't seem enough for a tuned 1275, I normally aim for 32-34 total

Interesting

I took the car for the rolling road, the pump was putting out 5-6 psi ( I have the mini spares hi flow float needles in and they seem to cope with the pressure) and its delivering half a pint in 15 seconds, approx.

When he ran the car on the rollers up to 5500 it was fine and produced a healthy 90 bhp at the wheels. I then asked him to take it to 6000 as I wanted the problem replicate the problem as it did. After scratching his head, Dave Bogg decided to change the plus to shorter one and this helped allowing the engine to rev cleanly, under load, to 6000 rpm. However when I took the car out it now does it at about 6250 rpm, not a big problem as I do not intend the rev it beyond that but I am a fanny and still want it fixed. Dave also observed a lot of ignition scatter at 3000 rpm dithering by about 10 deg around the 28 degree mark so, maybe this is the route of the problem?
When he first started the hydrocarbons were off the scale (I was already aware of this as I had richened it to try to overcome fuelling at the top end) but the CO at the hi end was very low, weak. This was down to me and the fact the needles was not seated correctly (embarrassed).
So now it running much better however when I change gear they still a slight hesitation and I feel that I have lost some torque at the lower end, about 2000 to 2500 rpm. This may of cause be because I had been running very rich?

Although not a complete success Dave did offer to give me a free session once I sorted the distributor out and I must say I was very impressed with his friendliness, willingness to have me around at all time taking photos and my constant barrage of question. For a 72 year old he is still very much in touch with both old style and modern tuning and would recommend him to anyone one with SU carbs. His cost is also extremely reasonable.

Edited by Petrolhead on Saturday 11th May 17:14

[Petrolhead]

PS, just been thinking, I am running a Powerspark Electronic ignition pack with a std Lucas coil, could there me a missmatch or could the engine be blowing the spark out