Since I bought this car it has never delivered full power. Above 3000 RPM, it “blubbers”, loses power and will not go further up the rev range. Pulling the choke out improves rev’s a couple of hundred, but not much change. I also get some weird brown smoke, or maybe it’s rust from the exhaust.
The car starts and idles perfectly, and runs nicely unless asked for full power.
I’ve tried everything I know. The car has been converted to HS8’s, no secondary butterflies
compression check – all 125
valve clearances all good
cam timing good
breaker points good, good dwell. Dizzy shaft not wobbling.
New dizzy cap
Timing good, at idle, and “all in” at 3000 RPM
Plugs gapped
Exhaust not blocked
Checked that carb type, needle spec, jet spec are correct for the car. Float level on spec.
Swapped in another coil – no change
Air filter box on or off – no change.
Fuel pumps seems to put out well; fresh gas
Can you think of anything I’ve missed, or should try?
Do not rev the old gal like that in neutral. My machinist so would have a hissy!!
MyLT1 had a hissy. Only once, and never again, I hope! Back fired and blew the air cleaner off. A good sized cloud of brown smoke from under the bonnett. Different for sure, but an indication of what the brown vapour is.
To me the video seems to show that the sliders in the carbs are not pulling all the way open!!! In a butter fly carb, that means the cable or link is too long??? You are not getting WOT!!
Several or all the intake cam lobes are “flat”. The intakes are not fully opening…
That is a way out idea!! One, maybe, but all???
As Carl notes the pistons, at least, not rising evenly. I would take the bells off the carbs and make sure the throttles are making it all the way to horizontal - the WOT position - and put thinner oil in the rear carb to assist it’s lifting. If it’s not rising fully it’s not fueling fully either. The choke out improvement might be a bit a hint in this direction? Paul
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What does that mean, Rob - and what is the advance in idle…?
Secondly; have you checked that the pedal has the full travel - and what about carb piston position at full throttle…?
Engine seems sound - lack of backfiring when it stagnates implies adequate felling. Ign advance problem should be pursued - do you have mechanical points or electronic ignition…?
The carb pistons were not rising equally, because after dismantling the front carb, I forgot to add damper oil. I use ATF. Now they rise equally.
The jet depth is about the same at around 100thou. This is deeper than other HD carbs I’ve owned.
Frank, the term “all in” refers to the total advance. I’m at 8 degrees at idle, and about 30 at 3000 RPM.
I tried a vacuum gauge today, and (according to a quick internet search) it suggests leaky valve stems, and the low readings rev’d up indicate early ignition timing. So, I advanced the timing to about 15 at idle, and no change. I’m new at using a vac gauge.
Got it. You said no secondary butterflies and I thought you had removed them.
I won’t offer any suggestions because IMO if you haven’t figured it out I know I never would. But in terms of data, have you read the plugs immediately after a period at WOT?
Your Vacuum Gauge video is interesting, (but not very encouraging)
I have a similar, but lesser issue, in that a vacuum gauge fluctuates rapidly about 1-2Hg, fluctuating around 17 at idle
I have isolated the problem as coming from the rear carb area, but inside the engine, by using a hose & ear (swapped in other HS8 carbs)…try the hose…do you hear a rhythmical sound?
I feel the engine does not develop full power.
I suspect a weak or broken spring, or worn valve guide in the rear 3 exhaust valves area
Your vacuum gauge readings seem worse than mine
In my case the nest step would be to remove cam covers and replace some springs, next would be head off examination.
My engine runs really quite well, others have counselled me not to mess with it
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4 causes are listed for this classic rapid fluctuations, Rob - to be pursued…
Ignition miss - doesn’t really sound like it at the revs you presented. Any signs of miss at low rpms - like in gear? However, some ign malfunction, other than timing, may be involved. Have you changed the rotor?
Do I have your solemn assurance that your ignition sequence is correct…? 1-5-3-6-2-4 (6-2-4-1-5-3 if you start at the front cylinder)
Blown head gasket - which should show up in the compression test. Which is not the case from your previous post…
Leaking valve - which should also show up in the compression test.
Weak or broken valve spring(s) - worth a closer look. One factor limiting top rpms for any engine in general is that springs cannot hold the valves in contact with the cams as revs rise. The valves starts to ‘float’ - and no further rpms rise is possible…
Normally; very, very few faults will limit the xk engines’ ability to rev freely in ‘N’. Having checked cam timing - did you notice anything unusual - or the timing chain tension?
A broken valve spring may be detectable in situ - possible by comparing compression forces between springs? But measuring spring length/strength would require head removal - which should only be a resort if nothing else is found. Springs don’t go weak at the drop of a hat…
Cry before you take it off - surely one can test the valve springs without head removal! The bottom end will hardly have anything to do with it. I’m out of ideas as well if that is of any help.
90 thou jet size??? I admit I don’t know much about those carbs but the 1.75" dual set-up H-6s on my (small) 3.4L uses .100 jets. But my 2L Datsun Roadster uses 1.75" SU clones with .090 jets. Are we sure those small jets are correct for the car?
Bob, I’ve been messing with mixture, and pulling the enrichment lever lots of times during testing, and the car hasn’t had a good run, so I think the state of the plugs is normal for this. Up close they don’t look too bad.
David, I’m looking for ways to remove the springs without removing the head; rope in the bores, compressed air. Years ago I watched Lou Fidanza use compressed air while installing GT25 cams and uprated valve springs to my car. I’ll consider it -hard!
Lee; I thought the jet seemed small too, that’s why I mentioned it. I’m trying to remember what my old HD8’s with UM needles had in it. But, as I said, these needles and jets are in the SU data for 4.2 Jag with HS8. I also remember having a 420 that would not rev unless the full air filter components were in place. I’ve never seen that with any car, but a fellow here (Merritt?) told me. Filter, or no filter makes no difference this time.
BCC is listed as a .100 needle, which is not narrow enough to fit a .090 jet until the 4th “station”. This according to the minty chart I just re-found. Looks to me like you have a mis-match. Further investigation is warranted here. As a reference, the wo2 needles on my 140 are .100" at the thickest(station 1), same size as the jet.
The biggest SUs I believe had .125" jets. Further, looks like the “UM” is, indeed, a .125" needle, fitting the big SU.
