Now, an air leak at the manifold would surely cause the mixture to weaken and have the reverse effect no? ie lower the revs? Any clues or ideas leading to a cure will be greatly appreciated.
SU woes
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brinylonshirt
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- Joined: Sat Apr 28, 2012 12:58 pm
- Location: uk
SU woes
Recently fitted twin HS2 SU carbs to my Imp. The carbs were checked and declared fit for purpose by a trusted long term user of the same SU setup on his 2 cars. The problem I`m having is getting the car to idle properly. All it wants to do is rev it`s head off. It has the correct jets/choke tubes and springs for this engine, but won`t play ball. The butterflies in both carbs are closing properly but it continues to run as if on wide open throttle. This is whether I have used any choke or not.
Now, an air leak at the manifold would surely cause the mixture to weaken and have the reverse effect no? ie lower the revs? Any clues or ideas leading to a cure will be greatly appreciated.
Now, an air leak at the manifold would surely cause the mixture to weaken and have the reverse effect no? ie lower the revs? Any clues or ideas leading to a cure will be greatly appreciated.
Re: SU woes
An air leak on a breather hose would indeed do this as you're admitting the air downstream of the throttle discs 
. Check the hoses for tightness and also for perishing bits, I had this exact symptom with a modified Toledo (SU binned and twin Stromberg SEVs fitted) a while ago, traced to a perished break in the hose to one carb.
. Check the hoses for tightness and also for perishing bits, I had this exact symptom with a modified Toledo (SU binned and twin Stromberg SEVs fitted) a while ago, traced to a perished break in the hose to one carb.J
"Home is where you park it", so the saying goes. That may yet come true..
"Home is where you park it", so the saying goes. That may yet come true..
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brinylonshirt
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- Location: uk
Re: SU woes
There are no breather hoses for the intake system on an Imp, at least no perishable ones. I see what you mean though and I`ll recheck the fanimold.
Cheers!
Cheers!
Re: SU woes
Are you sure the fast idle screws are off the cams at idle? Is the linkage set right so both carbs are roughly syncronised? Are both pistons free and dropping right down and did you "bridge" the jets and back them off 2.5 turns when setting the carbs up on installation?
Answer yes to all questions and the idle should be ajustable within limits for mixture RPM and balance.
Answer yes to all questions and the idle should be ajustable within limits for mixture RPM and balance.
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brinylonshirt
- Posts: 8
- Joined: Sat Apr 28, 2012 12:58 pm
- Location: uk
Re: SU woes
Yes x 3! That`s why it`s so infuriating. I guess it has to be an induction leak, but convinced myself that extra air can only have one effect: a weaker mixture and things slow down? Not the case here, it`s possessed I tell thee!
Re: SU woes
What did you do with the carbs' breather stubs then? Have you capped them?brinylonshirt wrote:There are no breather hoses for the intake system on an Imp, at least no perishable ones. I see what you mean though and I`ll recheck the fanimold.
Cheers!
J
"Home is where you park it", so the saying goes. That may yet come true..
"Home is where you park it", so the saying goes. That may yet come true..
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suffolkpete
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- Joined: Sun Jan 23, 2011 11:54 am
Re: SU woes
Has it got those overrun valves in the butterflies? If they're stuck open they can have that effect. Most people solder them up.
1974 Rover 2200 SC
1982 Matra Murena 1.6
1982 Matra Murena 1.6
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brinylonshirt
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- Location: uk
Re: SU woes
Do all SUs have breather stubs? These don't.
The throttle butterflies haven't got the overrun valves. They're the type with none fitted.
The throttle butterflies haven't got the overrun valves. They're the type with none fitted.
Re: SU woes
Well assuming that the hose wouldn't connect to the inlet manifold from the airbox as it does on the ex-Ranger van engine I've just checked with a fellow R3W/RoadsideRescue board member - who owns 3 875s, a Rootes Rebel (vertical 928cc version on twin Hitachi HS2 copies) and the Ranger which now has a bike engine in its place - then there must be something to pick up the fumes and your trouble is very likely a large air leak. Did you fit the same oil cap to the cam cover when you swapped the carbs across?brinylonshirt wrote:Do all SUs have breather stubs?...
Let it run for a short while and scoosh some of your ATF or brake fluid at it, when you can't see for the white smoke and the idle temporarily drops some, you found your leak.
:scratchin:
J
"Home is where you park it", so the saying goes. That may yet come true..
"Home is where you park it", so the saying goes. That may yet come true..
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brinylonshirt
- Posts: 8
- Joined: Sat Apr 28, 2012 12:58 pm
- Location: uk
Re: SU woes
It`s got the original crankcase breather from the side of the oil filler neck which vents to atmosphere, not into the air canister. I`ve tried the method you mention, has zero effect.