IP2 losing power

[gripper]

I cleaned the vacuum chambers of my IP2 out and re-assembled them prior to the NOC Devon Rally and gave it a bit of a polish and now it is a sick little puppy. (Didn't make it to Devon) It starts and runs and the tick over drops back when warm but it is reluctant to pull above about 3000rpm. Working on the premiss that "it was probably the last thing you were playing with" I've checked the carb slides are free and there is fuel getting through from the tank. The lack of power doesn't seem like a misfire or one rotor dropping out, almost like the slides are not lifting with the vacuum. Any thoughts?

[graham]

Could you have bent the tip of a needle?

[Richard Negus]

Dave,I presume you didn't take the carbs off the bike? I've seen a carb that didn't work after replacement because the gasket on the inlet side was upside down and the air passage was blanked off, stopping the piston working correctly. Modern gaskets seem to be symmetrical so it doesn't matter.What oil did you put in the dash pots? It should be SAE 40. Did you oil the pistons and damper rods when assembling?You haven't got a big spring left over by any chance?

[johnbirchjar]

Blocked main jet? Vac pipe sucking in air?

[gripper]

May have got to the bottom of it, as you know, most carb problems are electrical. My ignition trigger has mysteriously moved and is contacting the flywheel. Probably ok but the poor little thing can only take a certain amount of hammering before giving up the ghost. I'll get back to you all with the final result.

[gripper]

It's marginally better but still refuses to pull strongly. I didn't remove the carbs but did clean and oil the damper with 20:50 which is what was in there before. It starts easily the tick over is steady and there is good fuel flow to the carbs. The Ignition trigger was loose and the flywheel was striking it, but not now. (the big spring is in the dashpot) The carbs are pretty simple so I can't see what is likely to be wrong with them. Tank off time, again

[wust588]

Sounds to me that something is stopping one or both of the dampers from rising properly.Try just unscrewing the damper rods (but don't pull them clear of the carbs) and try to lift the dampers with the rod.Generally speaking, Rotella 40 or equivalent is the right oil for the dashpots.Make sure you haven't trapped some grit u/neath the dashpot flanges before you tighten the screws.I'm assuming the damper rods are screwed in square?

[gripper]

That thought had crossed my mind. I was meticulous in cleaning the flange and the piston does lift when not on the carb. That's my main line of attack. I might buy one of those ultrasonic cleaners and give the carb parts a "bath" I'm guessing that a lighter oil in the dashpots would allow the slide to rise and fall quicker. Would that mean quicker throttle response?

[gripper]

I've gripped the inside of the damper rod with my smaller bore measuring gauge and both slides move up and down freely. Curiouser and curiouser.

[Richard Negus]

My understanding is that the amount of fuel coming out of the spray tube depends, amongst other things, on the difference in pressure between that point and that in the float chamber. No pressure difference = no fuel flow; high pressure difference = large fuel flow.The underside of the vacuum piston is subject to ambient pressure ( or in our case, pressure in the frame plenum) by means of a drillway in the main casting through to the inlet joint face, gasket, and inlet casting.The upper side of the piston is subject to pressure in the gap between the bottom of the 'slide' and the carburettor bore; there's a hole in the flat bottom face of the slide part to transmit that pressure above the piston.When air is flowing though that small gap, its pressure is reduced and the piston rises until balanced by the spring; greater pressure difference = higher piston and more fuel flow.If the piston is not damped, the piston can rise too quickly when the throttle is snapped open such that the pressure difference at the spray tube actually drops and the fuel/air mixture becomes too weak to burn properly. The level of damping has little effect under steady throttle conditionsWith SAE 40 oil, cold, it is quite difficult to push the damper pistons down - unless of course they're worn.It may be that cleaning the pistons and chambers of accumulated gunge has reduce their mechanical friction such that 20W-50 oil is now not viscous enough.

[gripper]

The carbs have been off and are now clean inside and out, they weren't particularly dirty and there wasn't a great deal of deposit in the damper chambers when I initially cleaned them. The slides are free top to bottom. Tonights test ride reveals little difference, a bit of misfiring, initially pulls up to about 3000 then performance dropping away until crawling along the Shrewsbury bypass at 35mph. two minutes later, better but still not good. I'm beginning to turn my attention to the ignition trigger. It might have sustained some damage from the flywheel and be good enough to just run but little more. Strangely, when I got home (just) the heated grips had turned themselves on. It's normally a 5 second press on the button for that and I hadn't been near the button. I wouldn't mind replacing the ignition trigger but it's only been on there about 9 months and it's a right faff to change.

[gripper]

Referring to my earlier thread "loosing the will to live" it would seem that the trigger has a magnetic core. I know that magnets don't like to be knocked or hammered so i'm guessing the trigger is toast after taking a bit of a hammering from the flywheel. Looking as to why it worked loose, I suspect that the powder coating on the rotor housing went soft with the heat and the tension on the retaining screws was lost. Bo---cks

[gripper]

I've just fitted a new Boyer ignition trigger but the problem still exists. The bike continues to start and run with no problems the throttle will respond normally on the stand but when being driven it bogs down approaching 4000rpm. Last year when the problem was the ignition trigger, the symptoms were similar. The ignition unit was tested then and no fault found. Back to the carbs I think.

[johnbirchjar]

Could the problem be the "Advance & Retire" Isn't?J.B.

[gripper]

No John, I don't think that's the issue. The bike starts and runs and, once warmed up, the revs drop back commensurate with the retard circuit working properly.

[Anthony Duffield]

Gripper,I am not an expert and it took me some time to find the misfire on my Commander. If your bike was OK until you cleaned the carbs I would suggest that is where the problem lies.Does your bike pull strongly through the gears does it only bog down in fifth, if you drop it down a gear and give it hand full what happens? Is the problem load related, a weak mixture, getting too hot, partial seizure.Perhaps you need to go back to the start and carefully check face to face joints, gaskets and O rings on everything you touched when first cleaning the carbs.There is nothing more annoying or frustrating than a problem that seems obvious but is a cow to find.Good LuckTony

[gripper]

Hi Anthony, My initial problem appeared after I removed the suction chambers from the carbs for cleaning, at the same time I detected a knock on the LH side of the engine that I traced to the ignition trigger being struck by the flywheel after it had worked loose (the trigger that is) subsequently I have removed and cleaned the carbs completely and fitted a new trigger unit. The problem appears as I approach 4000rpm in any gear, but it is difficult with these engines and a helmet on to pinpoint the fault. I have had some improvement with different plugs but the symptoms are more like running short on fuel supply. Both taps have been open and make no difference. the carb slides are completely free to move from top to bottom of their range

[gripper]

I decided to carry out a more subjective road test to try and establish an accurate diagnosis. Accelerating normally would produce a misfire or distinct lack of power from about 3800rpm upwards irrespective of what gear was selected. Approaching 3500rpm and snapping the throttle open (not wide open) resulted in a clean acceleration up towards 7000. This would lead me to think that the rapid and large depression created downstream of the carb slide was enough to make the slide respond (under suction above the piston) to move the slide up correctly. Mincing around with gentle throttle openings is not creating enough suck to make the piston lift. This seems to indicate a restriction in one of the slides lifting cleanly or a blockage in the drilling delivering the suction from the engine side of the slide/piston. Might be getting somewhere at last. The petrol tank will know it's own way onto the bench soon

[gripper]

I've discovered a bent needle in the right hand carb which will need replacing. It's marked 51XA despite the workshop manual giving ADR. ADR appears on the SU/Burlen website but not 51XA The needle guide has a small lip to displace the needle forward in it's normal seated position. On close examination of my needle there is a distinct flat down one side. This could have been caused by the bend but also the skewed resting position will always have the needle hard up against the jet on one side.

[gripper]

Well the new needles are in and there seems to be no great difference. It still starts, runs, ticks over but has no urge and is flat above 4200rpm. I've done the carbs to death, the ignition trigger is new. now turning my attention to coils and ht leads in desperation.

[Richard Negus]

Dave,I offer this in the knowledge that involves taking the carbs off yet again - but have you checked the main jet bore? On a couple of occasions I've found a sort of lacquer fuel residue in there. Cleaning with a watchmaker's ream removed it without taking out any metal.Looking on the black side, maybe the rotor air passages are so blocked with carbon that it can't breathe enough air to run any faster. I think, further back, you said it only happened after some carb work so that it still the prime candidate.

[gripper]

Using the bike for work, the problem appears after about 8 miles, both there and back. going to have a look at the filler cap vent. Might be time to get the drill out

[gripper]

Nope, that didn't work. when the bike is started, it runs a little fast hunting around 1100rpm as you would expect before the retard switch kicks in. then after about 2 minutes the revs start to fall until the engine just stops, got to be petrol starvation hasn't it? Carbs off again, tonight, checked the float needles and seats, main jet and floats out, no fault found. both fuel taps are clear and flowing well, (no inline fuel filter fitted) further road test tomorrow morning. Tune in tomorrow for the next exciting episode.

[Interpol2471]

I had a very similar issue on my Classic that was thought to be fuel starvation and I found it was caused by using a silicon pipe to my fuel vacuum tap instead of the hard plastic pipe. At certain conditions the silicon pipe collapsed reducing the vacuum to the tap and therfore cut off the fuel supply.Just a thought...but if you have a soft flexi pipe change over to the hard type.

[gripper]

That eureka moment came last night after fitting two new coils. So what they say about 90% of carb problems are electrical is probably true. I'd been working on the principal that it was probably the last thing I was fiddling with. I've replaced the two 12v coils with two 6v coils but there is no ballast resistor in the system. Seems ok now. At least I know that I could eat my dinner off the inside of my carbs now and it did reveal a bent and worn throttle needle, now replaced. Thanks for everyone who spent their time with suggestions. No to get out on it and give it a damn good thrashing before winter sets in.

[gripper]

Premature celebrations still playing up. starting from scratch again.

[gripper]

Still struggling to get the IP2 to run properly. I've now established that when it is reluctant to rev (most of the time) pulling the choke out produces an increase in power. That would indicate the fuel that is coming through the enrichment circuit of the carb is making up for the lack of fuel at other times. Again I come back to the theory that the vacuum is not lifting the slide and needle, yet both are totally free to move. the vacuum must be leaking away somewhere. One possibility is the scavenge pipe from the frame plenum chamber. Any other thoughts?

[wust588]

Sounds like things might benefit from a richer mixture.Try a half a turn on each carb (clockwise), and see what the difference is.

[gripper]

Looks like I've got to the bottom of it at last. The left hand throttle bypass tube was not fully home in the rotor housing. Looks like when I refitted the carb I must have pushed the carb forward onto the manifold tube and bent the bypass tube. ( the RH bypass tube hides behind the primary chain case at the carb end and is difficult to see but in fact the bent one was on the left) Whilst there has been enough air/fuel mix to feed the engine at tick over and when revving up at standstill, calling for more power has allowed the vacuum between rotor and carb to fall to a level that would not lift the slide. The tick over is a little erratic still but that's seems to be down to the throttle butterfly not closing fully under the spring pressure. Thats been two months of "Rotating" missed

[johnbirchjar]

Well done young sir!! ,J.B.