Runs bad since oil change

JW240

Active Forum User
Coincidence I know, first oil change on the new engine. Ran it etc to check levels, starting popping out the exhaust, seemed odd so checked the plugs, rear 3 were black, really black but dry. Front were ok, maybe a touch lean. Cleaned and reinstalled and still popped abit. Checked the mixture and both carbs behave as if they are lean (lifting piston causes engine to stall and die) abit lost really, not sure why it’s just started running badly but the SU carbs don’t seem to make any difference when adjusting the mixture nut, not matter how rich, it stalls and dies. They are brand new SU out the box, have about 75 miles on them, SM needles on a slightly modded 2.4 (cam, manifolds, head work, cam etc)
 

jonbills

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If the carbs have only done 75 miles, there's every chance the bolts somewhere might have relaxed a bit somewhere and it's leaking air.
Other than that, check everything - does the chokes open and close equally and evenly, do they flow the same air etc.
 

JW240

Active Forum User
If the carbs have only done 75 miles, there's every chance the bolts somewhere might have relaxed a bit somewhere and it's leaking air.
Other than that, check everything - does the chokes open and close equally and evenly, do they flow the same air etc.
I did check all the bolts and things seem ok, the choke isn’t properly set, I just use wire for now. But ok choke it runs sweet and strong making me believe the lean condition. Just wouldn’t explain why the rear 3 plugs are black. Did the suck test with a tube and they seem decent, it ran well before today. That’s plugs 3 and 4 to show the difference
 

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Farmer42

Club Member
I have found that SUs will not run very well on the new E10 fuel. If you have been using that, try putting in an octane additive until you have used it then use the E5 super unleaded.

Also, check the float levels in the float chambers. This is critical to the smoothe running of SU carbs. You will need to carefully take the float lid off, turn it upside down an measure the distance between the float and the lid. Use a drill bit with the correct diameter for best results. If the distance is wrong, bend the metal tang until it is right. When you have done that, use an air flow meter to tune it to the correct specs. It will all take a bit of fettling and time but is achievable.
 

JW240

Active Forum User
I have found that SUs will not run very well on the new E10 fuel. If you have been using that, try putting in an octane additive until you have used it then use the E5 super unleaded.

Also, check the float levels in the float chambers. This is critical to the smoothe running of SU carbs. You will need to carefully take the float lid off, turn it upside down an measure the distance between the float and the lid. Use a drill bit with the correct diameter for best results. If the distance is wrong, bend the metal tang until it is right. When you have done that, use an air flow meter to tune it to the correct specs. It will all take a bit of fettling and time but is achievable.
I’m on e5 so hopefully no issue there

Funny you mention about the float levels as I had them off abit ago to clean them. Wonder if I could have knocked something when reinstalling them
 

SacCyclone

Club Member
At the bottom of each carb is the main jet which fattens up when the choke is pulled.....sometimes they will stick like the choke is still set, shouldn't on a newer carb but you never know.

With the choke off, reach under each carb and push up on the main jet to make sure it has returned up and is not running fat.

If that is ok i would move on to the floats like others have said.

Mike
 

JW240

Active Forum User
Update, took the carbs apart today, floats looked fine. Needles weren’t sticking, cleaned it all up but still really bad. Runs horribly and won’t rev at all. Put on choke it runs amazing. So confused, especially as it’s out the blue. Changed the plugs too and it keeps making the back 3 absolutely black almost immediately. Kinda scared something serious is wrong
 

Rob Gaskin

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Update, took the carbs apart today, floats looked fine. Needles weren’t sticking, cleaned it all up but still really bad. Runs horribly and won’t rev at all. Put on choke it runs amazing. So confused, especially as it’s out the blue. Changed the plugs too and it keeps making the back 3 absolutely black almost immediately. Kinda scared something serious is wrong
I'm sure nothing serious is wrong if it's the back 3 it will be the carb. However if it runs better with the choke that is confusing me.
 

jonbills

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I suspect the problem is the front 3/front carb.
It's not unusual to have black plugs, esp if you're using the choke.
 

JW240

Active Forum User
I suspect the problem is the front 3/front carb.
It's not unusual to have black plugs, esp if you're using the choke.
Me too, but why suddenly. And why would running it on choke solve he running issues? I’m really stumped
 

Rob Gaskin

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You say it's a new engine? Is the inlet manifold for cylinders 1-3 bolted up tight - I'm wondering it it's drawing air?
 

JW240

Active Forum User
You say it's a new engine? Is the inlet manifold for cylinders 1-3 bolted up tight - I'm wondering it it's drawing air?
Appears to be, haven’t found any vacuum leaks there. It also ran good for the first 50 odd miles
 

SacCyclone

Club Member
Still sounds like main jet sticking on one of the carbs....it will run better with the choke on......just double check.

Check brake booster to see if it failed and is drawing air into the system.....just plug the vacuum hose to brake booster to confirm
 

JW240

Active Forum User
Still sounds like main jet sticking on one of the carbs....it will run better with the choke on......just double check.

Check brake booster to see if it failed and is drawing air into the system.....just plug the vacuum hose to brake booster to confirm
Have checked and rechecked the carbs, pistons run smoothly in their bores.

I will double check the vacuum to the booster, booster is only a year old so hoping its not that.
 

Rob Gaskin

Treasurer
Staff member
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Have checked and rechecked the carbs, pistons run smoothly in their bores.

I will double check the vacuum to the booster, booster is only a year old so hoping its not that.
Hope you aren't offended here but just to clarify in case there is a mis-understanding, the main jet moves independently downwards of the piston when the choke is used. Your reply suggested to me that you were checking the pistons? Sorry if you were just adding that.

Hope you get this bad running sorted soon.
 
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