20 replies to this topic

[mkw]

Hello, 

 

I've got an issue with hesitation when accelerating. I can slowly build the revs up and it's fine but if i want to set of quick or put my foot down the car just bogs down, it's been happening for a while and i want to get to the bottom of it.

 

I've set the valve clearances, plug gaps, timing and fuelling with a colourtune.

 

I'm running a 1991 998cc, Stage 1 - SW5 cam - HIF38 with LZX1509 damper and ABD needle (currently 5w30 oil in the dashpot but i've tried 10w40 and 20w50) - electronic ignition on a Lucas 59d distributor - NGKBP6ES plugs

 

Hopefully someone will have an idea what i'm missing!

 

Thanks,

Michael

 

[absx2]

Do you use the original air box or a cone filter ?

It sounds like the carb piston is rising too quickly under load, over fueling, air leak from servo or breathers for a start.

Has this problem just occurred or has fitting a stage one kit caused it ? 

[Johnmar]

Damper oil should be ok the way as you describe it.

Can it be a coil/dizzy relayed? I had some issues back in the days. Acceleration was only possible when doing it slowly and taking the time otherwise it did not take anyload. Swapping the dizzy was the answer that time.

Maybe something to rule out aswell

Edited by Johnmar, 27 December 2020 - 08:13 PM.

[mkw]

Do you use the original air box or a cone filter ?
It sounds like the carb piston is rising too quickly under load, over fueling, air leak from servo or breathers for a start.
Has this problem just occurred or has fitting a stage one kit caused it ?

I use the original air box, I do have a cone I could try though. I just didn’t like the extra noise when I used it. What do you mean by air leak from servo? The crank case breather is connected to the carb, I could try blocking it?

The stage 1 kit has been on the car around 10 years so shouldn’t be that.

One thing I have noticed by pure fluke is that if I cover the air intake and restrict it slightly I can Rev the car fine. I found out when I had my hand blocking the air filter intake.

[mkw]

Damper oil should be ok the way as you describe it.

Can it be a coil/dizzy relayed? I had some issues back in the days. Acceleration was only possible when doing it slowly and taking the time otherwise it did not take anyload. Swapping the dizzy was the answer that time.

Maybe something to rule out aswell

Dizzy is an accuspark one, new about 18 months ago - what’s the best way to test it? Vacuum advance works when I suck the pipe.

I’ve not changed the coil for a few years, do you know a test? I’d rather not just buy things and it turn out not to be that. I assume you measure ohms?

Edited by mkw, 28 December 2020 - 08:09 AM.

[absx2]

OK so the car will bog down when revved from under the bonnet but is OK when you cover the inlet ? that says it`s running too lean or the vacuum advance has pushed the timing too far.

With the car parked try richening the mixture and see what happens then turn the dizzy with the engine running and see what happens, sometimes setting the timing by ear gives better results that a timing light.

If that doesn`t bring any joy remove the servo banjo bolt and block it with your thumb and give it a few revs to eliminate a possible air leak.

Don`t block the crankcase breather, it needs to be there but check the pipes are good and the filler cap is on ok and has a 3mm hole in the underside. any extra breathers will require more fuel as they will be pulling more air into the inlet.

As Johnmar has said, it would be worth checking the dizzy weights and springs are still where they are supposed to be and if you have a waxstat jet make sure its not loose.

Edit:  Too much compression and low grade fuel will give the same issues, if you have a City E motor with 10.3:1 CR they are fussy

Edited by absx2, 28 December 2020 - 09:42 AM.

[mkw]

OK so the car will bog down when revved from under the bonnet but is OK when you cover the inlet ? that says it`s running too lean or the vacuum advance has pushed the timing too far.
With the car parked try richening the mixture and see what happens then turn the dizzy with the engine running and see what happens, sometimes setting the timing by ear gives better results that a timing light.
If that doesn`t bring any joy remove the servo banjo bolt and block it with your thumb and give it a few revs to eliminate a possible air leak.
Don`t block the crankcase breather, it needs to be there but check the pipes are good and the filler cap is on ok and has a 3mm hole in the underside. any extra breathers will require more fuel as they will be pulling more air into the inlet.
As Johnmar has said, it would be worth checking the dizzy weights and springs are still where they are supposed to be and if you have a waxstat jet make sure its not loose.
Edit: Too much compression and low grade fuel will give the same issues, if you have a City E motor with 10.3:1 CR they are fussy

I’ll have a go a richening the mix a little and try that. There is no servo on my car, I removed it years ago. The inlet just has a bolt in place now.

I’m running a HIF38 so the jet shouldn’t be an issue, could I need another needle?

When I checked the timing it was 32 BTDC at 4000rpm with the vac disconnected and plugged which gave 15 BTDC at 1000rpm idle.

Could the carb spring effect anything? I do have another car running a HIF carb that I could switch to try?

[cal844]

Set the timing by ear, in 10 years of minis my dad has never used a timing gun.

My friend who always uses a gun has always had poor running cars which need revved as soon as they're started due to the timing being so far out.

Loosen the clamp bolt just enough so you can rotate the dizzy.

Set the idle to 2000 rpm vacuum disconnected and blocked.

Listen to the revs of the motor as you turn the dizzy, they'll peak and drop off. Hold the dizzy at the peak then back slightly. You also need to listen for pinking as you are aware.
Lock the dizzy clamp and reset idle.

Now turn the engine off and try restarting. The engine should go straight to the set idle speed smoothly and without needing throttle.

Give the engine a short burst of 4000 rpm whilst listening for pinking.

If you hear pinking at this stage you need to retard the timing slightly.

The engine does sound lean as you are able to get the car to respond better by covering the air inlet.

Hope this helps.

Cal

Edited by cal844, 28 December 2020 - 11:41 AM.

[floormanager]

You can test for lean by puling out the choke a little.  If it improves the response then you may need a different needle.  Read before and was told that stretching the damper spring slightly may help with mid range lack of go.

 

Doesn't the mixture screw just set the idle mixture and the needle controls the mixture over the rev range?

Paul

[sonscar]

I am not familiar with HIF carbs but with HS carbs the mixture adjustment has an effect throughout all the rev range.Steve..

[mkw]

Thanks for the suggestions, that’s tomorrow sorted! I’ll play with the mix, check the coil and dizzy then also tweak the timing by ear - I double check with the gun and report the figures.

[Tornado99]

Have you done the carb dashpot slider drop test to check if it still has good fitment?
With dashpot off carb, seal the air passages underside of dashpot with masking or electrical tape. Hold dashpot rightside up, with slider supported inside by a finger. Release the slide and count how many seconds it take to drop fully. Repeat a few times. If it happens to fast or too slow, this will cause problems when changing engine rpm. The SU tuning guides list appropriate drop times some place. Also a few recent youTube vids showing this and other SU tests:

 

 

 

At about 22 min mark on this very detailed vid:

https://youtu.be/e15QlVLjJiU?t=1350

Edited by Tornado99, 28 December 2020 - 06:45 PM.

[cal844]

Thanks for the suggestions, that’s tomorrow sorted! I’ll play with the mix, check the coil and dizzy then also tweak the timing by ear - I double check with the gun and report the figures.

If the car is road legal give it a test along a fixed route, go by feel or the stopwatch

[mkw]

Have you done the carb dashpot slider drop test to check if it still has good fitment?
With dashpot off carb, seal the air passages underside of dashpot with masking or electrical tape. Hold dashpot rightside up, with slider supported inside by a finger. Release the slide and count how many seconds it take to drop fully. Repeat a few times. If it happens to fast or too slow, this will cause problems when changing engine rpm. The SU tuning guides list appropriate drop times some place. Also a few recent youTube vids showing this and other SU tests:




At about 22 min mark on this very detailed vid:
https://youtu.be/e15QlVLjJiU?t=1350

Thanks for this, plenty of information there. I’ll do the drop test first as I’ve woken up to snow this morning so there won’t be any road tests yet 🙁

[mkw]

Just a quick update, due to weather I’ve not been able to take the car for a spin.

I have done the drop test and it takes 3.5-4 seconds ish which is less that the SU website quotes so I tried the HIF off my other mini and it’s about the same.

I have got a yellow spring to try though which may help if the piston is lifting too quickly. The yellow is 8oz compared to the standard 4.5oz red.