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Post by xian911 on Dec 26, 2023 16:11:02 GMT 1
I try to find why this 4L0 doesn't run as expected... Backfiring on left side between 3.000 & 4.000 . Not reving above 6OOOrpm. Same behaviour with 2 sets of carbs(4L0 01) except rev ceiling at 6k or 8k depending on carbs used . The carbs used in video come from another machine running well. Already swap CDI. Crankshaft oilseals changed. What can cause such behaviour? reeds? stator? Air intake? What else? Thanks for your ideas. Christian Backfiring at 20''. Rev ceiling at 1'05"  VID_20231226_120326 by xian911, on Flickr |
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Post by reedpete on Dec 26, 2023 18:10:37 GMT 1
Maybe the High speed coil on the stator.
Unplug the brown/black/red connector under the seat and measure the resistances between the different wires. Search of ‘stator resistance’ should throw up old references.
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Post by abar121 on Dec 26, 2023 18:40:58 GMT 1
If that doesn't give any clues, do leakdown and compression tests.
Check grounds / plugs / coils / exhaust baffle.
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Post by muttsnuts on Dec 26, 2023 18:54:18 GMT 1
will be stator, either pickup (most likely with your symptoms) or low and high side windings
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Post by steeley on Dec 26, 2023 19:17:36 GMT 1
Stator had the same on my LC350 some years ago . Checked carbs / put another coil on /spark plugs / cdi . Maybe not in that order .Like the bike i am getting old .
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Post by howard on Dec 28, 2023 20:01:22 GMT 1
Had the same problem with my 4L0 recently,I tried 3 sets of carbs and they all were slightly different but it wouldn’t pull above 5.5k , until I tried my new Allspeeds and it now revs out.
Not going down the caustic soda cleaning method again, it didn’t work and I ended up with a red stripe across my face all Xmas where I got splashed and just wiped it .
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Post by abar121 on Dec 30, 2023 21:40:10 GMT 1
I had similar with my rebuilt H1 500. It would miss on one pot, even after switching the carbs, coil, plug etc. Timing, compression / leakdown was perfect.
It was some crap in the baffle that got left over from decoking with caustic soda. A real headscratcher as I thought they were spotless inside.
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Post by xian911 on Jan 5, 2024 16:27:42 GMT 1
Still in trouble  I measured resistances (stator, coils,...) all are in tolerances. Same measurements on second set of coils, stators, CDI, rectifier. Had a try with clean 31K exhausts, no change. Equal compressions on both sides and within tolerances. Swapped spark plug wires. Had a try with another coil. Backfiring on left cylinder between 3.000 and 4.000rpm dissapears when choke is on. could it be a bad wiring loom? |
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Post by stusco on Jan 5, 2024 18:35:25 GMT 1
Mutts mentioned the pickup have you checked it ?
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Post by 4l04ever on Jan 5, 2024 19:06:37 GMT 1
Swap the carbs side to side and see if the fault moves. You have to fit a long choke pipe but should work...
If the fault swaps sides, it is the carbs
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Post by xian911 on Jan 6, 2024 15:32:35 GMT 1
Mutts mentioned the pickup have you checked it ? just done... (Pickup coil (white/Red to black wire - 87 ohms +/- 10% at 20c Low speed windings - 271 ohms +/- 10% at 20c High Speed windings Also set the air gap between the pickup and flywheel at 0.40mm (was 0.60mm) Big difference of exhausts temperatures between left and right (exit of cylinders) 80°C again 180°C No change. |
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Post by midlifecrisisrd on Jan 6, 2024 15:40:49 GMT 1
Funny that it goes away with the choke on 🤔
Have you done a leak down test?
I've had engines leak between the crank area and the gearbox
Also check the crank is 180 degrees between left and right TDC in case it's twisted
Steve
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Post by xian911 on Jan 6, 2024 17:11:51 GMT 1
Funny that it goes away with the choke on 🤔 Have you done a leak down test? I've had engines leak between the crank area and the gearbox Also check the crank is 180 degrees between left and right TDC in case it's twisted Steve I also begin thinking that the crank is twisted I also begin thinking that the crank is twisted  That's the only thing I haven't checked yet. Engine was rebuild, new conrods, bearings, seals, pistons, rebore ...but centre assembly wasn't checked. |
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Post by midlifecrisisrd on Jan 6, 2024 17:29:08 GMT 1
Now then 🤔
It should never really be a problem but I'm sure somebody years ago had a bike that wouldn't run right (may have been at mutts) but the only thing that was done was changed the labyrinth seal
Couldn't see anything wrong with it but had tried everything and nothing else sorted it
Best check the crank with a dial gauge and a degree wheel then do a leak down test
As said I've had 2 leak into the cases. 1 was a set of tdr cases that looked like someone didn't know about the 9th m6 bolt from underneath and had been levering with a screwdriver and the other was the lesson in not using fresh three bond. Stuff had set solid before I'd got it torqued
Steve
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Post by xian911 on Jan 6, 2024 18:01:00 GMT 1
As said I've had 2 leak into the cases. 1 was a set of tdr cases that looked like someone didn't know about the 9th m6 bolt from underneath and had been levering with a screwdriver and the other was the lesson in not using fresh three bond. Stuff had set solid before I'd got it torqued Steve The symptoms were the same before rebuilding...hence different three bond same result...cases matched and well torqued Only causes remaining : twisted crank or bad labyrinth although I heard that it is very rare. I 'm also wondering why a failed labyrinth will only affect left cylinder ! |
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Post by reedpete on Jan 6, 2024 20:05:34 GMT 1
I have also been thinking about a slipped crank across the last couple of days.. be worth quickly checking that out if only to rule out..
most likely some faults attributed to failed lab seal, were also due to slipped cranks…
crank spilt to replace the seal and problem vanished when reassembled ( correcting the un detected slip error) ..
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Post by chippy348 on Jan 6, 2024 20:28:21 GMT 1
Not seen a slipped crank for years, the main factor in the whole "twisted crank" came from a bad batch of crank webs that were machined wrong, around 500 of them.
Not going to go into specifics' but they ended up being used and twisted cranks were a thing. This was back in early 2000's most have been changed/scrapped.
Unless the centre mains have gone the the lab seal does not come into contact with anything and cannot wear out.
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Post by reedpete on Jan 6, 2024 20:51:50 GMT 1
Not seen a slipped crank for years, the main factor in the whole "twisted crank" came from a bad batch of crank webs that were machined wrong, around 500 of them. Not going to go into specifics' but they ended up being used and twisted cranks were a thing. This was back in early 2000's most have been changed/scrapped. Unless the centre mains have gone the the lab seal does not come into contact with anything and cannot wear out. Is that a different topic from standard cranks that have been reassembled a couple of times and then have centres with in sufficient interference fit friction to stop them indexing ? |
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Post by muttsnuts on Jan 6, 2024 20:57:56 GMT 1
it was me who had a failed lab seal, but that was on an RGV250 (the lab seals are sh*te and can fail due to the ethanol in the petrol)
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Post by chippy348 on Jan 6, 2024 21:14:30 GMT 1
Not seen a slipped crank for years, the main factor in the whole "twisted crank" came from a bad batch of crank webs that were machined wrong, around 500 of them. Not going to go into specifics' but they ended up being used and twisted cranks were a thing. This was back in early 2000's most have been changed/scrapped. Unless the centre mains have gone the the lab seal does not come into contact with anything and cannot wear out. Is that a different topic from standard cranks that have been reassembled a couple of times and then have centres with in sufficient interference fit friction to stop them indexing ? In my experience we dont often build cranks with all "used parts" IE we use the centre shaft but fit new innder webs, OR we may use the old inner webs and use a new center shaft, but either way we check the interference fit. Hope that makes sense |
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Post by chippy348 on Jan 6, 2024 21:16:34 GMT 1
it was me who had a failed lab seal, but that was on an RGV250 (the lab seals are sh*te and can fail due to the ethanol in the petrol) They have a rubber outer to the seal, was it a Japan seal ? |
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Post by 4l04ever on Jan 6, 2024 22:16:55 GMT 1
A twisted crank would affect the right cylinder not the left, as the timing for ignition is based on the left cylinder.
Try swapping the carbs side to side.
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Post by xian911 on Jan 6, 2024 23:12:50 GMT 1
Try swapping the carbs side to side. Will have a try but I already try with another set of carbs without sucess. |
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Post by 4l04ever on Jan 6, 2024 23:24:09 GMT 1
If you have tried known good carbs then it is not them.
How about reeds?
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Post by xian911 on Jan 6, 2024 23:41:54 GMT 1
I tried with reeds + inlet rubbers from a 250lc(4L1) without any change. |
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Post by muttsnuts on Jan 6, 2024 23:43:01 GMT 1
it was me who had a failed lab seal, but that was on an RGV250 (the lab seals are sh*te and can fail due to the ethanol in the petrol) They have a rubber outer to the seal, was it a Japan seal ? not sure Paul as it had all disintegrated when I got the crank out so couldn't tell what make it was |
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Post by muttsnuts on Jan 6, 2024 23:44:59 GMT 1
ok back to basics, what's the compression pressures for each cylinder - I know you said "within tolerances" but what are the pressures ?
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Post by xian911 on Jan 7, 2024 0:07:22 GMT 1
ok back to basics, what's the compression pressures for each cylinder - I know you said "within tolerances" but what are the pressures ? I just checked on pictures I took and now I have a doubt  being within tolerances... 75 psi on both cylinders. Too low isn't it? Not sure I did it correctly (not full throttle) Will redo it tomorrow. |
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Post by abar121 on Jan 7, 2024 11:06:04 GMT 1
It needs to be done with the throttle wide open.
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Post by 4l04ever on Jan 7, 2024 11:24:09 GMT 1
Leak down test?
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being within tolerances... 75 psi on both cylinders. Too low isn't it?