16 replies to this topic

[Ricester]

Hey,

 

I am still having issues with running in my newbuild. I am trying to tackle two problems -

 

1. The temp gauge goes to "hot" within 5-6 minutes of idling. 

2. I cant get the car to "sit" when idling. Idle is all over the place and if down to 1,000rpm, will cut out. 

 

I attach a couple of links to videos which will help explain things better than I can. 

 

https://youtube.com/...w?feature=share

 

 

Video 1 the car has been run for about 5 minutes. Sequence has been - choke all the way out and it starts at about 2,000rpm. As the car begins to warm up, the revs climb up. They continue to climb unless I put the choke in a little. I put the choke in a little to bring the revs back down to 2,000 or lower and again they climb within a minute or so, back up. They do this until I have the choke fully in, when the choke is fully in its still idling (up and down) around 2,000rpm. 

 

If I press the accelerator sharply and let it go again, throttle response is good but it "holds" the revs, for quite some time. If I rev it to 3,000rpm and bring my foot off the accelerator instantly, revs will stay at 3,000 for 5 seconds or so and slowly and inconsistently drop back down. Sometimes its longer. 

 

At "idle", if I can call it that, revs move about, but constantly climb. sometimes they dip down quite a bit, then pick up but overall it slowly climbs. 

 

Please ignore the temp gauge. I was attempting to by-pass the voltage stabiliser to determine if it was the issue re "overheating" - was it simply telling me it was overheating without it actually overheating, but despite testing that second gauge prior to installing, it decided part way through to give me a zero reading, initially working. Ive no idea why. Another thing to sort. 

 

The second video is the carb off the car. 

 

https://youtu.be/f6yt8s5542M

 

I have a spare carb. Its in worse condition than the one in the video but regards the piston, it moves MUCH more freely. The carb in the video is the one on the motor at present. Apologies for the crap quality of the videos, Im hoping the noise more than anything can assist with the first video. 

 

Does the piston move satisfactorily or is it "sticky"? It moves consistently but it is much slower than the second carb I have, both with the dampers in. 

 

Is there anything obvious in the videos thats causing the horrible revving? Timing has been done with a strobe to 12 degrees btdc at 2000rpm (approx obviously!)

 

Ive got a colortune kit coming to sort out the mixture and Ive got a compression kit coming as its something Ive never checked. Im really struggling to nail down what the issue is here. 

[cal844]

Sounds like something is gummed up in the carb, or a sticky accelerator cable....

Carb piston looks okay. Is there oil in the dashpot?

Edited by cal844, 01 May 2023 - 07:41 PM.

[imack]

Air leak or badly worn throttle shaft.

Edited by imack, 01 May 2023 - 07:46 PM.

[PoolGuy]

Air leak or badly worn throttle shaft.

Agreed. Also the piston isn't dropping quick enough IMO.

[Ricester]

Thanks all, 

 

There is nothing gumming up the Carb, it has been stripped down twice now by myself and cleaned inside and out. The accelerator cable isnt sticking. There is oil in the dashpot, the SU recommended oil. Without the damper and oil the piston moves extremely freely, straight down. With the damper and oil, its as per the video. 

 

Re an air leak - Ive never checked for that and I have little experience in general around this - how do I check for an air leak? Are you talking about the carb specifically or anywhere else? 

Re the badly worn throttle shaft - are you suggesting the same thing, i.e. an air leak around the throttle shaft? Or something else? Again how do I test for this? 

 

I know everyones busy, it would help me greatly if I could get some more in-depth analysis, unfortunately Im not experienced enough to understand fully when its just a few hints at something. 

 

Thanks 

[Steam]

With regard to the temp rising you really need to verify what the temp actually is at the top hose / tstat housing. An ir gun would do it if it is below @ 90C it is ok nd the gauge is not correct.
Double check the timing and advance for peace of mind as these will cause both problems. And check the vac advance in particular as it will affect both idle and introduce an air leak in one go.
Check for air leaks around the carb body, manifold, everywhere on the intake using a lpg handheld bottle with a thin hose around all ar3as. Alternatively you can spray carb cleaner around all spindles, joints and the manifold. The revs will jump if a leak is present.
Hope this helps.

[cal844]

Try a heavier oil, like air tool oil or 10w40 engine oil

[lordcakes]

Ref testing for air leaks, spray something (carb cleaner, brake cleaner, wd40 etc) around the area that you think might be leaking. If the revs rise when you do this you have an air leak.

One thing worth a try, I've just had a problem with a rough idea and I thought it was the throttle spindle but I found that the leak was from the main suction chamber. Removing the 3 screws and a thorough clean there solved the problem. Might be an option if you have recently dismantled or replaced the needle.

[Ethel]

The ignition being over advanced could cause over heating & a poor idle.

[PoolGuy]

I know everyones busy, it would help me greatly if I could get some more in-depth analysis, unfortunately Im not experienced enough to understand fully when its just a few hints at something. 

 

Thanks 

Check out AC Dodds videos on Youtube, there's some stuff on SU's that you'd find useful.

[sonscar]

To rev up it needs air.If the butterfly is shut it is coming in from somewhere else.This tends towards being weak a symptom of which is unstable idle.Start at the head face and work towards the air filter carefully looking for leaks.Check and not guess.This sounds too bad to be throttle spindle or minor adjustments.Steve..

[Ricester]

Thanks all for your input. I have quite a number of theory’s. Here goes…

1. Testing an air leak using carb cleaner there’s a jump in revs when spraying around both sides of the butterfly shaft. The revs then stay high for 5 seconds or so and slowly come back down.

2. No real effect spraying around the spacer. Vacuum pipe and fittings doesn’t seem to leak.

3. Was hard to tell around the manifold - there was no big jump but bear in mind the revs are constantly moving around without my carb cleaner input.

4. This is a new build that hasn’t been on the road yet. I’m not convinced the head gasket (copper) has sealed. After I turn the car off I’m smelling a sweetish smell. I don’t know that it’s burnt coolant but it’s what I think it would smell like. No white smoke to speak of.

5. All of this is done with the choke out at around 2,000 rpm. If I push the choke in, it dies. I have the carb jet adjusted (HS4 wax stat) two full turns down from full lock, if you know what I mean.

My analysis of ALL of this is:

1. There’s an air leak around the carb butterfly shaft.
2. Head gasket may not be sealed properly.
3. The biggie - I think my compression ratio is whack. I have a 998 a+ block bored to +20, and a 12g295 head. The head has been at the machine shop with the block, and has been converted to unleaded and had some work done around the valves etc but given I’ve never talked to the machine shop about it, I don’t think compression has been considered. I haven’t had it skimmed. I certainly haven’t considered it, by the time I understood compression ratio more I’d already had the head on the car.

This may sound silly but I just need it confirmed - would too low a compression ratio cause the car to struggle at below 1,500rpm idle? Would a poor compression ratio also contribute to overheating? I’m not saying it HAS poor compression but with what I’ve described so far, are these all indicators of it?

Time to take bits off again. I plan on contacting AC Dodd about the carb work required unless anyone has any better ideas.

I’d much appreciate if anyone has any advice at this stage.

[Ricester]

Also, should this shaft be blocked off???

Attached Files

•  B25676C8-E7F7-488A-B3F9-3F1FEC4216E2.jpeg   41.72K   5 downloads

[imack]

That needs blocking or connecting to the closed circuit breather. Left open like that means you have a massive air leak!

[Ricester]

That needs blocking or connecting to the closed circuit breather. Left open like that means you have a massive air leak!

Hahaha, excellent!! Well I feel like the novice that I am now, but thanks!