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rough idle, then races to 4000 rpm?? help!


Jedi Trainee
Country flag
First off - Hello to all. I have been just reading and digesting all the info so far.
I'm in the process of re-restoring my '64 MkII. The car was bought new by my Dad, and I began restoring it when I was 13. Finished my senior year of HS, drove it ~2K miles and parked it when the speedo cable broke..went off to college and left it in his garage.
Present: Now in my garage and I drained the gas tank (put it away "wet" in 1991...I removed and cleaned the fuel bowls (ugh) and sprayed gumout into the carbs liberally.
I got it to start, but the idle is rough, without opening the throttle, but working the choke a little, it will run, but it tends to go from a barely running idle, then without warning, it revs above 3500 almost 4 or 4.5K rpms..at which point I am moving as fast as I can to get to the ignition switch to shut her down..
So..what you guys think? I took the fuel pump off, disassembled and cleaned. It seemed OK once cleaned. I am guessing I need to bite the bullet and tear the carbs down.
Any other ideas in case it is still doing this after the carb cleaning?
Sorry for the long post..


Luke Skywalker
Check the carb pistons for sticking, I bet they are gummed up.
Pull the dashpots off, lift the pistons out and clean the needles and piston with carb cleaner.
Do NOT use any abrasives on the needle or piston.
Also clean out the inside of the dashpot.
Mark each pot to piston so you don't mix them up and fill the pots with oil.


Jedi Warrior
What shape are the hoses in that go from the fuel filter to the carbs and from carb to carb? What shape is the Fuel filter in?


In addition to what Spritenut said, take a pipe cleaner and inspect each end to make sure there are no sharp wire burrs on it. Once you know there's nothing that will scratch the jet tube, take the vacuum chamber, piston/needle, and spring off the carb (marked so you can put them back in exactly the same position). Spray carb cleaner down the jet tube and on the pipe cleaner, then gently work the pipe cleaner up and down in the jet tube to remove and/or dislodge the deposits there.

Since you parked it wet, there are almost certainly heavy gum deposits in the jet tube and on the needle. The deposits can both block fuel flow and prevent free movement of the piston/needle assembly. When you pull the choke out you're giving it the additional fuel it needs to keep running, but the air/fuel mix isn't right and things won't allow stable running.


Jedi Trainee
Country flag
Ok..so after the first post, I went and pulled the dashpots and took the jet lines off (from bowl to needle). Blew them out with Gumout as I replaced both a few weeks back with new.
The needles were very clean, the one carb was almost out of oil. Shot carb cleaner all over. wiped down the needle itself, shot the piston with cleaner..basically cleaned everything, but was a bit disappointed not to find more 'gunk'.

Re-assembled. Car started after the fuel bowls refilled. It runs better..now I don't have to fidget with the choke to keep it running, but it is running pretty rough and lopes a bit.
It did rev once (I touched NOTHING) again up to 4000, but then quickly comes down almost as if gas starved, but will "idle" roughly at ~600 rpm.
I actually saw what appeared to be fuel mix come out the carbs a few times (wrong way!!?) Yikes. Like the valve timing is off..? This car ran like a champ when I parked it..I tend to think it is not something mechanical..but rather a loose or gunked up..(?)

BTW: Fuel filter is new, installed today. Steel gas line appears ok..I used the pump to push a bit of fuel into a bucket, good flow and clean. All rubber fuel lines were installed in the last month.

I appreciate all the feedback. I'm going to head back out and keep tinkering..I know I'll find it eventually. I'm starting to wonder now if it might not be in the electrical side..? timing perhaps. Will let you know what I find.
Oh, and merry x-mas to me..my ignition light is now on...it wasn't doing that before. Oh well..it was easy everyone would do it!!


I don't wish to point out the obvious but just to be sure...

"I went and pulled the dashpots and took the jet lines off (from bowl to needle)"

You did go through a complete carb adjustment after that didn't you? Taking the carbs apart as far as you did requires you to recenter the jets and to re-set the air/fuel mixture. My pipe cleaner method was suggested on the assumption that you didn't want to disturb the tuning of the carbs. My method isn't applicable now.

If you took the carbs off to do this work you'll also need to (and probably should anyway) check the airflow balance between the carbs. A complete carb tuning is now in order.


I would also check the needle valves in the float chambers. If they are sticking, you will gat too much fuel and make it race as you describe!(Check for gunk in the float chambers too as it would likely deposit ther when stored "wet"!)


Jedi Trainee
Country flag
Doug, Thanks and no worries..its not obvious! I did reassemble..and did not re-tune yet. How does one center the jets? The old workshop manual doesn't mention how to do that one.. I was going to try the pipe cleaner and I thought "geez, might as well clean it all and be done with it."
So, off to the search engines to find a good SU tuning procedure. I found one once, but apparently didn't bookmark it. (doh!!)

Excellent point. I did take them apart before I drained the tank and refilled...but it is possible some gunk got in there since then as I failed to clean the fuel pump until after I ran the car (Brain-f@rt). I'll tear 'em apart again tonight and check/clean. thanks again!


Jedi Hopeful
Just curious... When you said "put it away wet" do you mean gas sat in the tank since 1991?

If so I would spend the money and have the tank cleaned out by a radiator shop...

If there was gas in the tank for a long time it would turn to varnish - sticky stuff... And even if you drained the tank there will be residual gunk (ugly black stuff - I have pictures) in the tank. This will mix with any new gas creating something that is is almost like gas - but not really... Cars don't run very good on this stuff...

When I first got my car I just drained the tank of the old gas put in a couple of gallons of new gas; shook the car as best I could then drained that gas out (it didn't look too bad), so I just put in some new gas and went from there... Bad move on my part - ended up having a valve stick, so had to do a valve job a lot earlier than planned.


Chris... what an excellent link! Thanks for posting it.

64RockSprite, Googling for links like Chris posted is a start but eventually you'll need a couple of other printed manuals if the one you have doesn't have information like how to center jets. At this point you need to start with all the basic settings (assuming everything is free and 100% clean at this point) and do a complete carb tuning. Obviously you need to make sure fuel is flowing from the tank and that your filters are clean also.

The first LBC I worked on was a '64 MGB and it had been parked for about a decade with gas in it. I used some pretty strong paint stripper in its tank to remove varnish and gum that was completely blocking the pickup tube in the tank. What a mess.


Jedi Trainee
Country flag
First off - THANKS CHRIS! As I type this I am killing a few trees printing out that manual.

cd and Doug - I'll drop the tank tomorrow night. Why I didn't come to that conclusion on my own..no idea. Too ready to get it back on the road perhaps?
Now that I think of it, I did see one small piece of black goop come out of the rear jet last night when I cleaned it on the bench.

Great stuff everyone, really appreciate the help. I'm honestly a bit embarrased I did not think to clean the tank itself..everything else was dirty....why wouldn't it be too?
I'll get back to you and let you know how the tuning went once I get my 'house in order' (clean) first.
Thanks to all,
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