Any ideas why my XK120 should be gently surging at idle once warmed up? Engine has just had a full rebuild and is currently running on points with vacuum advance disconnected (for now). I plan to swap the points system for a positive earth electronic trigger to replace the mechanical points. Recommendations as to which make/model to use with Lucas sports coil would be welcome.
Engine is basically as for standard XK150 with 8:1 compression, B-Type head.
Thanks,
Chris
hmm. need a bit more detail Chris. are you using a standard xk 150 engine and inlet system with thermo choke? are there any modifications? if there is a brake servo where is the vacuum take off?
also need to check the accelerator linkage is free to move and that any engine movement is not causing the throttle to open slightly.
Why sort of rpm surge are you experiencing?
Thanks Phil. Engine is the original early XK120 block fitted with early B-Type head (as for XK150, Mk 1 3.4, Mk VIII) - standard cams and valve sizes. Rebuild involved re-bore to +.040", new Mahle 8:1 pistons, crank grind .010" under, balancing. Carbs are its original Nov 1950 H6 thermo type. No vacuum servo (still on drums). The carbs I donāt believe have been touched, just bolted back on. I have TL needles ordered plus new jets and various other carb bits such as needle valves and nitrile floats. I havenāt checked the surge on the rev counter, but itās probably only a couple of hundred revs - will check again tomorrow. Engine seems lovely and smooth (carefully balanced) otherwise, with no visible movement as it idles. I did notice when I re-started it after the first run and the car had been sitting a while (so was probably quite hot) that it displayed quite major surging which settled down after about 15-20 secs or so. I just assumed I had started it without waiting for the pump to properly fill the float bowlsā¦
After reading several discussions on this site, I think Iāve decided to stick with points and condenser - at least I understand them!
Cool. Sounds pretty much like my car. I rebuilt it as standard but the head casting was porous so I used a btype. I also stayed with points and condenser with the distributor rebuilt.
I cant offer much advise at this point but suggest you get a few miles on it and carefully record what is going on especially with water temp etc. Please keep us updated.
With best regards
Philip Dobson
i was about to suggest to stay with stock type points distributorā¦but you said itā¦yesā¦understand and roadside fixable. At least use until all else is sortedā¦so you donāt have to add another variableā¦and a harder to track one at that. I would suest you test a spark plu wire for ohm resistanceā¦you donāt want anyā¦should read less than 1 ohm. Hi resistance modern wires can have VERY hi suppression for modern electronic module cars with HEI distributor/coils. Simple test is that a no resistance wire will lite up a simple 1.5 V continuity test liteā¦a hi resistance wire will not. (just to eliminate one more hard to track issue).
Nick
Thanks, Nick.
Spoke to David Lonsdale today who suggested it could be due to small air leaks around the throttle spindles as the engine moves about slightly at idle. I guess a carb rebuild is on the horizon!
My plug wires are copper core and I have just fitted Lucas 78113A suppressor caps, which make a really positive connection to the plugs - even soldered the split brass washers to the ends of the plug wires which are then held down by the acorn nuts. They look great, too! Currently using NGK BP6ES plugs, but may go to BP5ES.
Chris
a little quick start spray in the throttle shaft area will tell you if an intake air leak is present. LITTLEā¦or carb cleanerā¦a bit less volatile. yup the 5 is a bit warmer pluā¦6 colderā¦depends on engine condition, tuneā¦and hiway or cityā¦
In a message dated 8/13/2018 2:54:26 PM Pacific Standard Time, noreply@jag-lovers.com writes:
chrislackner
August 13
Thanks, Nick.
Spoke to David Lonsdale today who suggested it could be due to small air leaks around the throttle spindles as the engine moves about slightly at idle. I guess a carb rebuild is on the horizon!
My plug wires are copper core and I have just fitted Lucas 78113A suppressor caps, which make a really positive connection to the plugs - even soldered the split brass washers to the ends of the plug wires which are then held down by the acorn nuts. They look great, too! Currently using NGK BP6ES plugs, but may go to BP5ES.
Chris
Yes, thought I might try that, Nick. I think I will probably change to BP5ES as I do mostly running at lower revs. Boy racer days are behind me!
Chris
David rebuilt my carbs. Superb job.
Easy check: while running, point an unlit propane torch at the throttle spindles: if engine gains speed, youāve found the problem.
Sadly, due to ill health, heās not rebuilding carbs any more. Might possibly go back to it next year.
Oh my goodness thatās a shock. David rebuilt a set of etype carbs for at the beginning of this year.
Dave recommended Southern Carburetors to rebuild mine, amongst one or two other specialists. Today I found I could actually feel slight movement in the throttle spindles when I wiggled them, so figure that is most likely the cause of revs fluctuating by something a bit under 100 revs.
If you can feel side-to-side movement in the throttle spindles, youāve likely found the issue.
Joe Curto is hard to beat!
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I would be very concerned about SC rebuilds
Ohhhā¦ thatās a thumbs down then! I can do everything except the spindle re-bushing myself, in fact a load of parts arrived today from Burlen. Do you have any recommendations, then, Phil?
I noticed today, after about a 90 mile run, when I restarted it after having turned it off for a few minutes, it exhibited the same major surging (probably up and down by 1000 revs) for about 20-30 secs before it settled down - weird! Temp gauge was on about 90 degs at the time. On the move itās between 60 & 70 degs, in town about 70-80. But of course the engine must still be somewhat tight only 150 miles after the rebuild. Iām using one of Guy Broadās recommended multi-blade plastic fans which Iām hoping will keep the temps within bounds. I really dislike electric fans cluttering up the front of the radiator, so am trying to avoid going down that route.
Itās all about our personal experiences. I would speak to burlen first. Unless David has recommended a similarly skilled individual.
07714 759198
Hi Chris:
Reading your posts about the āsurgingā it sounds very similar to a problem I once experienced. Many years ago I belonged to a club who occasionally put on slalom events on a large parking lot. One was timed individually around a pylon course and never got out of second gear while using a lot of revs (in fact I found it easier to launch in second given the length of the Moss box throw from first). Anyway, in the summer when pursuing this activity naturally the car got pretty warm and sometimes on starting it would exhibit the symptoms you describe. It went away after 20-30 seconds. Someone at the time described it as fuel boiling/vapourising in the inlet manifoldānot sure if that is technically accurate with respect to what was occurring or not. Do you have the thick gaskets between the carbs and the manifold? I seem to recall these were fitted to overcome similar problems on start up when the car is hot. Just a thought.
Regards,
Chris.
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Hi Chris,
Iām very interested to read your opinion, because I was wondering if something like that could be the cause. The car never did this before, but after the engine rebuild it came back without the spacers between carbs and manifoldā¦ I have just purchased some even thicker ones (12.5mm) in the hopes that this will improve things.
Will report back on results.
Chris
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Had the carbs off and apart yesterday.
First off, I couldnāt use the new 12.5mm heat spacers between the carbs and manifold because they require longer studs, which I donāt have. So they went back on with 3 ordinary gaskets on each, which might help the surging problem a little bit in the meantime.
Left piston was not dropping cleanly like the right one, and a bent needle was found to be the cause. I replaced the existing RF needles with TL ones, and also replaced the jets. Both pistons now drop with a nice clunk.
Replaced both floats with the āunsinkableā closed cell plastic type. Someone had already replaced the needle valves with Viton tipped ones, so these were left as is.
Found that one of the cork sealing washers in the jet assembly had been replaced with a rubber O-ring, so that was remedied.
Both spindles are noticeably worn, as movement can be felt, so they will be re-bushed in the near future.
With everything put back and with the jets simply adjusted to 2 1/2 turns out from fully screwed in, the engine ran noticeably better than it had before. Fine balancing and mixture adjustment will improve things even further, but I will wait until the spindles are done.
Itās nice to make some progress!
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