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Coughing and spitting


Dave the tram
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Have discussed this before, but one last search for ideas before I get new head as a last resort.

My mk3 GT6 developed tendency to hesitate and spit back through carb when pulling hard. Now had rebuilt carbs, new fuel pump and some pipes/fuel filter, electronic ignition etc. Starts first time and runs well almost all the time. Can accelerate full through gears to 5k revs, and less prone to the problem, but still hesitates and spits back after pulling hard in higher gears for a while, especially uphill. I'd decided it was valves, compression bit low on rear cyclinders (rear carb sooty from spitting) and doesn't hold compression well. Old leaded head. But it hesitates and looses power before it spits, which suggests fuel to me, but no problem with supply to carbs. Could it even be wrong jets causing this as it has K&Ns and a sport rear box. What jets should it have and will I be able to check what mine are - a 'specialist' did the carbs and set them up.

Cheers

Dave

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KN will certainly make i weak at full depression , there are few needles available for strombergs , suggest fitting std air box and see what that does 

 

plugs will breakdown under load and give a good kick but not loose power till t does it , sounds weak 

 

you could find a stronger damper spring they are graded by the wire diameter.or add a heavier / thicker damper weight as other ideas to try

Pete

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It  your GT6 is a late Mk3 then it has weak needles to start with. I would talk to Burlen Fuel Systems, 01722 412500, about the richer needles and stronger carb springs. Though be aware the range of needles is now limited for Strombergs.

 

If richer needles are not available then fit stronger carb springs anyway.

 

Dave

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Pete, Dave

I think you might be on the right track. I'd been topping up with 3 in 1 so switched to engine oil and probably overfilled them a bit. Drove hard once up to temp and seemed better, could only get it to cough after driving a while and constant full throttle in 4th from 65-70. Could try gear oil! But I think damping only affects sudden throttle whereas this is at steady full throttle, unless some sort of fluttering happens.

 

It is late mk 3, and they have blanking plug on base so I guess these are CD3 with jets adjustable by special Allen tool. Like I said I got someone to rebuild them for me, I did once have the top off to check diaphragm previously but didn't check needle and jet type. Can you see down dash pot if they are Allen key type after removing oil? I suspect that adjusting the needle to make richer isn't the same as fitting different needles. It does currently give good mpg - about 30 overall and driven quite hard, 40 on a long journey cruising at 70.

 

Think I'll phone Burlen's tomorrow about jets. If I look at current jets is the type stamped on them so that I know what's currently in them?

 

Many thanks

 

Dave

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Dave, engine oil is recommended for dashpots. There are early and late jets and needles for mk3. Both needles are top adjusting.

 

I bought a re-furb kit for my late mk3 carbs from Rimmers. The kit contained jets but no needles, which I ordered separately. On checking I discovered Rimmers only sell one kit and it has early jets! So I had to order those as well because they wouldn't change parts of a kit. I hope there has not been this sort of mix up with your rebuild.

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The dashpot is a damper to richen mixture on acceleration , like a pump on a downdraft

 

3 in 1 is cats pee and the damper wont work, if some clown says its better then other things are wrong innthe first place

 

you will have temperature compensators fitted bet the things are partly open when they should be fully closed, if open at idle at normall temp you set the idle mixture too weak and its then weak across the full range

 

take them off , fit two new o rings and screw the nut on the bimetal strip to close the plunger

if it never opens ,,, good. Or block the bypass competely

 

its designed to weaken the mix when hot , as a emission device and bleeds air around the throttle plates

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Right, they do have the temp compensators, but while having a look round found a big split in rocket cover breather just where it joins one carb, so fixed that but not run it yet. Can't believe this is the whole problem as its been coughing for ages and can't believe the man that did the carbs put them back with a dodgy breather. Can you explain a bit more Pete, blow by blow, how to do the trick with the the compensators.

 

Cheers

 

Dave

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Held on with 2 screws on the side , they have a plastic cover again held on with two small screws

 

have a look at buckeye triumps techy pages excellant how to on carbs , ignor the models the basics are common

 

you can set these things scientifically if you so desire the main problems is they are reliable but go out of

Of controll with age

 

the temp sender has two o rings one on the outer face one down its bore these get crappy they are in the

fitz all repair kits

 

the bimetal strip responds to temperature and opens a short platic plunger which allows air to by pass the closed throttle plate, there are holes and ducts inside the carb body which pass to the manifold , the manifold to carb gasket has holes in to match ,fit them upside down and you block cancell any bypass

simplze. But once you have the blighters in your hand you will see the qplunger and the small nut which tensions the plunger onto a simple seating

 

most will find the plunger is a rattling good fit and sits away from the seat, so you need to tighten the small nut this loads the plunger get it nicely shut , light finger prodding will show its sitting shut.

 

the idea is when engine temp is very high it opens and allows a bypass of the idle setting and speeds up and leans the mixture.

 

if its open when setting idle mixture you never get it correct , its far more important its shut than it being

set 'correctly'

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Pete

Great, that all made sense, took the covers off the temp sensors, found the nuts and tightened them both down fairly firmly about 1.5 turns on one, 2 turns on the other. Can't really test drive for a few days cos (a) here up north they keep chucking nasty salt all over the roads, and (B) despite all my ingenuity I can't get the new steering mounts to stop the rack from sliding across the car. A full on test drive is probably not wise - if only I could get the steering rack as secure as that bloody window handle I've been trying to shift for weeks..... But that's a different thread!

Cheers

Dave

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  • 3 weeks later...

SUCCESS!!!!!

After a couple of years tracking this down, it's running like dream. Because I thought I'd done everything else, including 'professional' carb rebuild and set up, I was saving up £1200 for new head including getting the stubborn old one off. Instead spent £12 on stromberg adjuster tool! Half a turn clockwise on each needle did the trick - it was running lean. Started, ran fine cold and hot, idles fine and drove hard for half hour with no coughing. 70 to 80 full bore, then full throttle in 2nd and 3rd all the way up Winnats Pass - half a mile of 20% climb.

I might just leave well alone now but I'd be interested to know how each carb is set, one needle was stiffer to turn than other. Do they get stiffer as you reach the limit. I'm told I mustn't turn to far anti-clockwise cos they fall out, but is there any reason not to go fully clockwise, counting the turns, then back to see if set equal? Do they just reach a stop when fully up?

 

Now it goes, and steers, might go mad and spend the money on better tyres or fancy wheels.

 

Happy man - thanks for the guidance Pete and all.

 

Dave

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Its often the basics that get out touch and the wallet drives the solutions into desperation

 

bdst to remove the air piston and set the needles small plastic washer to an equal setting on each piston

like almost flush with the base

how stiff depends on the spag washer tapped in to hold the needle nut assy down.

 

So set the needles mechanically in the same place then adjust together if needed

There are a number of variations that means they realy dont have to be set exactly the same but close

 

glad its running better and saved the myths of deep pocket syndrome

 

remember these carbs would at the car age probably been flow tested before fitting but earlier wouls all be just set to predetermined mechanical settings and whacked on and they worked ,

no messing with mixtures or balancing .

 

 

you get get less problems with std needles ,springs and std air box with a cold air tubes

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if you remove the top cover and pull out the piston and spring you will see  down the dashpot oily bore at the bottom is a hex key nut arrangement which is held in place with a sprag washer

best way to visualise this is on the buckeye site  but you have to remove all the adjuster assy and needle to get at the markings on the head of the needle

as these are CDSE i doubt anyone has ever attempted this without some moral support  !!!  and i would hazard a guess that you have std needles  , there are very limited alternatives available   so....    unless you realy have tot   dont go there !!!!!

 

even searching around getting the real whats supposed to be fitted is a chore 

 

here a good read 

 

 http://www.buckeyetriumphs.org/technical/Carbs/AdjNeedle/AdjNeedle.htm

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  • 2 weeks later...

Held on with 2 screws on the side , they have a plastic cover again held on with two small screws

 

have a look at buckeye triumps techy pages excellant how to on carbs , ignor the models the basics are common

 

you can set these things scientifically if you so desire the main problems is they are reliable but go out of

Of controll with age

 

the temp sender has two o rings one on the outer face one down its bore these get crappy they are in the

fitz all repair kits

 

the bimetal strip responds to temperature and opens a short platic plunger which allows air to by pass the closed throttle plate, there are holes and ducts inside the carb body which pass to the manifold , the manifold to carb gasket has holes in to match ,fit them upside down and you block cancell any bypass

simplze. But once you have the blighters in your hand you will see the qplunger and the small nut which tensions the plunger onto a simple seating

 

most will find the plunger is a rattling good fit and sits away from the seat, so you need to tighten the small nut this loads the plunger get it nicely shut , light finger prodding will show its sitting shut.

 

the idea is when engine temp is very high it opens and allows a bypass of the idle setting and speeds up and leans the mixture.

 

if its open when setting idle mixture you never get it correct , its far more important its shut than it being

set 'correctly'

Hi Pete - Ive followed your advice to Dave and now got round to shutting off the temp compensators on the CDSE 150's on my Vitesse Mk2 - Ive screwed down the 3/16" nut - just checking this is the correct procedure see pic 1  - Also noticed the rubber washer on pic 2 has all but disintegrated - just a year . Pretty sure I cant find one of these so was going to make a cardboard washer with a smear of Hylomar Blue 

 

Best regards

Paul 

post-1431-0-90089600-1489932985_thumb.jpg

post-1431-0-05031200-1489933028_thumb.jpg

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That will work its there to stop air being sucked down the sides

obviously made of self destruct after market self funding rubber

 

you could block off the port completely , leave the comps on as dummies in case a nosey mot guy

might just reckon youre supposed to have them

 

Pete

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