20 replies to this topic

[Tonylamb]

put the car on the rollers today. runs beautifully higher up the rev range but I have an annoying flat spot. when changing gear there is a hesitation as you go back on the power. This happens even if the revs don't drop below 4000 so it doesnt seem like the cam is the problem. i've tried different dash pot oils etc. the fueling is great and she made 96.1bhp at the flywheel at 6600. 1330 engine fast road cam. Vizarded HIF 44.

One thing we noticed today is with the strobe on the timing when its at about 15 degrees of advance at idle. But when you blip the throttle instead of a steady increase in the advance it flies up to about 30 degrees settles back and then moves steadily. Is it possible a weight is loose or perhaps a spring has gone. or perhaps the weights/springs are too light? any thoughts as to if this could be the source of the flat spot.

[ACDodd]

The flat spot when you change gear is the damper piston. It’s drop rate is too slow.

Ac

[Tonylamb]

Hi AC. I've tried different dashpot oils etc. How can I improve the drop rate of the piston? Stronger spring perhaps.

The advance thing is odd but I didn't think it would cause the flat spot.

The carb has been vizzarded. some meat taken out of various parts to improve flow.

Any suggestions of how to sort the problem would be appreciated. The fueling in general was really good. took a bit out of the needle to richen the mid range a tiny bit.

[luismx123]

Hi AC. I've tried different dashpot oils etc. How can I improve the drop rate of the piston? Stronger spring perhaps.

The advance thing is odd but I didn't think it would cause the flat spot.

The carb has been vizzarded. some meat taken out of various parts to improve flow.

Any suggestions of how to sort the problem would be appreciated. The fueling in general was really good. took a bit out of the needle to richen the mid range a tiny bit.

youre going to have issues setting up the carb since its been vizarded. SU carbs are finely balanced and any shaving or removing of metal will have an effect on the finely tuned things like the dashpot piston to wall clearance etc. Changing spring wont necessarily change the drop rate as its used to adjust the height:rpm. oil is there to adjust the initial jump that happens once the throttle is opened. Have a look if you can find a different damper - I know AC has made a post here about a quick drop version. 

[Cheeser]

LZX 2085 Is the fast drop type that is fitted to MG metro and others.
I would have thought that if only minor flowing done on carb should not create a problem if it has been properly set up for mixture and timing.

[Tonylamb]

I can't tell which plunger is in it beccause it doesn't seem to have any ID. Its also been played with I think to make it drop faster. A good deal of metal has been shaved from the base of the damper in order to improve flow. This isn't wrong in itself because the flow on the carb above 3500 rpm right up to 7000 is great.

but its a case of being careful what you wish for I suppose. I think i might prefer to have less ultimate power and get rid of the flat spot.

At the moment if you are at say 4500 and you come off the throttle and go back on hard you get a mementary pause before it goes again. its irritating but not incorrect.

Graham at CCk said the flat spot was possibly caused by the flowing of the carb damper. my post was really to check if the advance on the distributor which is a bit odd could be part of the problem. i don't think so because at 4000 its pretty well advanced anyway and doesnt drop that much. As always I'm likely to go with what AC says. The issue is how to fix it. New carb perhaps. may go for twin 1.5's not as much ultimate power but possibly a better drive. or maybe another 44 which has more torque i'm told. Can I perhaps swap the damper and the pot over and see what happens?

[GraemeC]

Was the 'Vizarding' done by RetroMinis by chance?   

I've never seen one personally, but have a look through the forum, where I am sure there are older posts where people have had issues with these carbs.

[sonscar]

AC has it spot on,not surprisingly,lack of acceleration enrichment.With EFI this is tricky to dial out,with a carb??????Steve..

[ACDodd]

https://www.facebook...0153741647/?d=n

Ac

[Tonylamb]

It was but I wouldnt blame them for bad workmanship or anything. The carb makes a ton of power at the top of the rev range as Ive said. It would appear that a vizzarded carb isnt necasarily any good for road use. That said it flat spots at 4000 rpm so it wouldnt be much good for a twisty track either?

The question is what do I do now? I have another 44 somewhere I could swap the damper and pot and needle over and see what happens.

[luismx123]

https://www.facebook...0153741647/?d=n

Ac

your link doesnt seem towork :/

[ACDodd]

Try it now

Ac

[Tonylamb]

AC i can get hold of a pair of HS4's on a decent manifold. There are so many opinions of twins v HIF 44. I like the look of the twins and I don't believe they are as hard to set up as people say. I had HS2's on a 1275 before and they were great.

I dont think that I'm going to be able to get rid of this flat spot because I suspect it is caused by the carb. Not that the carb was badly done its just not I think right for the driving I'm doing.

Would you switch to twin HS4's in my position. Your opinion is of interest to me.
TW

[Tonylamb]

I should have said that I'm not that bothered about ultimate top end power. I rarely hit 7000rpm. But I would like decent low end torque and good mid range acceleration. Fast road driving is nothing like track work IMHO>

[ACDodd]

Twins always perform better if you can get them setup properly, and there aren’t many people left who can do it properly. The flat spot you have between gear changes is easily fixed and is nothing to do with having the carb flowed. See the link I posted.

Twins have to be in freshly rebuilt or new condition to get the best from them.

Ac