12 replies to this topic

[Rorymc]

95 spi converted to 1400cc + carbs
Twin weber 45 dcoe

I have converted my spi to twin weber carbs, after alot of set up to get it two tick over wright, I went for a drive. The take up after hitting the trottle is fine but it takes it along time to rev back to normal and surges a bit. I was thinking it could be throttle cable but there is slack on it and the cable isn't getting stuck. I don't know if it is anything to do with the adjuster bar on throttle that can be shortened/lenghtened. Since the car does go back to tick over slowly makes me thinks it has to do with the butterfly valve in the carb as the carb snaps back shut instantly. So i don't know whats going on

I need help but haven't found any other thread that covers this, thanks for any help

The adjuster bar is on the wright of the pic

Edited by Rorymc, 17 February 2012 - 06:55 PM.

[bmcecosse]

Twin 45 DCOE?? Very much a full race mod - let's see a picture............

[Kerrin]

If you hold the throttle linkage shut will the revs die down quicker? If. Or is it an air leak, are the correct rubber gaskets used, with give in them to allow flex? There are internal springs in the manifold side of the carb which may be broken or missing.

Are the bottoms of the carb bodies intact with no cracks, as cracked grub screws can cause erratic running.

Are the floats in good condition and not filling with fuel, this is also a common problem.

[Rorymc]

Kerrin,
It revs slow back down even if I hold it shut. i think you nailed it with not allowing enough flex for the rubber o rings to seal.
I tightened up the Carb to Inlet manifold to tight and that is causing an airleak that only leaks after i rev it up and the revs fall slowly as the leak stops as the vacum decreases.

I will take it off again tomorrow and check the rings and put it back with enough flex.
Hope it works, I reackon ur wright. Don't think the carb is cracked but i will check

Thanks man, I will let you know if it works

It's only one carb, I meant twin barrel. The pic has a different throttle, I have a proper one on it now

Edited by Rorymc, 18 February 2012 - 10:11 PM.

[sonikk4]

Are you using Misab seals or the old plastic mount with seals either side. Have you over tightened the spring washers??

http://www.demon-twe...fse-misab-plate

Aah just seen you have the plastic seal retainer fitted. Not good, i had these on my twin 45's on my RS2000 and had no end of trouble. Went over to the Misab plate type seal and no more issues with air leaks.

[Kerrin]

I agree, misab plates are the way forward, I can't tell whether you have the spring washers or rubber (cosworth) washers behind the carb mount nuts but IIRC you need a 25 thou gap between the springs or cups to give flex to the mount.

Try backing the nuts off till you have the clearance and it may help.

Air leaks can also cause backfiring on the overrun, and big blue flames out of the exhaust on my old mini running a 45 but we learn from our mistakes!

[Kerrin]

Also a second spring on the throttle linkage on the left hand side of the picture may help, it may make the throttle stiff but may help figure out the try problem.

Edit, read your post again and you have changed the linkage!! Ignore me.

Edited by Kerrin, 19 February 2012 - 12:19 AM.

[Rorymc]

Yea when I was putting them on they look **** to me. Just bought a set of misab from ebay.

Thanks lads it sound like an airleak alright. It backfiring a little too. I will back them off to see if it helps until the others arrive.

I'm glad it's only simple anyway

[pogie]

I had a similar problem with my DCOE setup when I first fired my new engine up. It was very slow to return to idle and would hunt up and down with the throttle closed. I traced the problem to the little plate that came with the Weber linkage kit that bolts onto the end of the throttle shaft. Even with the tickover adjustment screw removed it was still holding the butterflies slightly open and I only found this out when I held the carb up, looked down the chokes and I could see light around the edge of the butterflies.

It was only the tiniest gap and it took me ages to find as I tried changing the timing, idle jets, idle screws and float height. A couple of mm removed from the linkage plate sorted the problem and the engine ran perfectly.


So maybe not the same cause as your problem but it sound like the same issue with too much air at idle speed.

[Rorymc]

I had a similar problem with my DCOE setup when I first fired my new engine up. It was very slow to return to idle and would hunt up and down with the throttle closed. I traced the problem to the little plate that came with the Weber linkage kit that bolts onto the end of the throttle shaft. Even with the tickover adjustment screw removed it was still holding the butterflies slightly open and I only found this out when I held the carb up, looked down the chokes and I could see light around the edge of the butterflies.

It was only the tiniest gap and it took me ages to find as I tried changing the timing, idle jets, idle screws and float height. A couple of mm removed from the linkage plate sorted the problem and the engine ran perfectly.


So maybe not the same cause as your problem but it sound like the same issue with too much air at idle speed.

I have the carb off again so i checked to butterfly valves, with the idle screw out they are fully closed. Just waiting on mestabs and cups off ebay

Cheers anyway

[Rorymc]

Yea it fixed straight away after fitting misab plates, they are far better seal and after I hit the throttle it is now revving back down fast.


Thanks for all the help lads

[sonikk4]

I would second having a second return spring as it will make a big difference in throttle control. I modded my old setup and it gave a nice firm feel to the pedal and snapped the butterfly's nicely.

[Kerrin]

Glad you got it sorted.

K