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OK I give up, it's got me stuffed [edit - now 95% fixed!]
https://www.ausmini.com/forums/viewtopic.php?f=2&t=52985		 Page 1 of 4
Author:  sgc [ Thu May 07, 2009 10:44 pm ]
Post subject:  OK I give up, it's got me stuffed [edit - now 95% fixed!]
This f@#$ing van has got me stuffed, I can't figure out what's wrong with it. I'm ready to throw in the towel...

What's good: It starts first time every time, and could idle for Australia at 700-800rpm. At low speeds (road and engine) it runs beautifully, smooth as silk although not terribly powerful (it's a stock 998 after all).

What's bad: It'll cruise at 80km/h in top at light throttle just fine, but push harder and the engine develops a stutter which is most obvious in the exhaust note, but the power loss can be felt too. It tops out at about 100-110km/h, and sounds and feels awful with anything more than very light throttle at this speed. Try to go faster and you end up going slower due to the misfire.

I've changed just about everything I can think of in pursuit of this problem:

  • New Bosch GT40 coil
  • New points, set to .016"
  • New condensor
  • New dizzy cap
  • Centrifugal advance lubed and moving freely
  • No runout in the dizzy shaft that I can see or feel
  • Known good set of plug leads and plugs, gapped at .030"
  • Rocker clearances set at I think .014"
  • Fully rebuilt HS4 carb -- new float valve & needle, new .090" jet, new ADF needle, new butterfly disc and shaft, new gaskets throughout, unknown spring. No vaccuum leaks that I can find.
  • Currently running SAE80 gear oil in the dashpot -- I've tried everything, this produces the best results
  • Timing set at about 8deg BTDC at 1200RPM
  • Idle mixture just showing a hint of yellow at 1200RPM by the colourtune
    [edit]
  • New fuel filter
  • Brand-new-looking air filter


I'll be buggered if I know what else to check. Currently I'm thinking perhaps there's an obstruction in the exhaust somewhere -- I'm wondering whether some f***wit has had his fun with a can of expandafoam as I can only get 13L into the 27L fuel tank when dry, and if this is the case I wouldn't be surprised to find the muffler full too.

Any other suggestions? Otherwise my next move will be a trip to Minitopia after payday...
Author:  simon k [ Thu May 07, 2009 10:52 pm ]
Post subject: 
isn't the plug gap meant to be more like .025" ?

what does the colourtune look like at 4k rpm?
Author:  paul_ewins [ Thu May 07, 2009 10:55 pm ]
Post subject: 
I've never had that problem in a Mini, but my old 1300 Barina twice had similar issues. Once was a blocked fuel filter and the other was a collapsed catalytic converter. Of course your Mini won't have a Cat, but your idea of a blocked tailpipe or muffler would be the same. Along the same lines as the fuel filter would be gunge in the fuel lines or a faulty fuel pump.
Author:  sgc [ Thu May 07, 2009 11:03 pm ]
Post subject: 
simon k wrote:
isn't the plug gap meant to be more like .025" ?

what does the colourtune look like at 4k rpm?


I've had the plugs at .025" and .035" with no discernable difference. A bigger gap is supposed to ignite the mix better if there's enough energy to spark at that gap, so that's why it's at .030". I run the 1152 in the other car (with a 123 dizzy) at nearly .040".

Colour at high revs (can't tell exactly, the test tach doesn't go that high) at light throttle is quite yellow, that's probably the heavyweight dashpot oil.
Author:  sgc [ Thu May 07, 2009 11:06 pm ]
Post subject: 
paul_ewins wrote:
Once was a blocked fuel filter


Forgot to mention that one, it's new too.

paul_ewins wrote:
Along the same lines as the fuel filter would be gunge in the fuel lines or a faulty fuel pump.


With the idle mix set leaner or with a lighter oil in the dashpot the symptoms set in lower in the rev range, but are lessened with application of the choke. This I think points towards there being enough fuel in the float bowl.. if there wasn't, asking for more fuel with the choke wouldn't work.
Author:  niceboy1275 [ Thu May 07, 2009 11:17 pm ]
Post subject: 
My clubman was just like that... After cracking the sads and giving up and running it to a fantastic mechanic, he changed my air filter..... Back to normal again... I felt like a bit of a moron....
Author:  74snail [ Thu May 07, 2009 11:17 pm ]
Post subject: 
With mine we reset the tappets,rechecked the spark plug gaps, checked the points,advanced the timing a little, we then checked the accelerator and choke cables-the accelerator cable needed to be adjusted so that it opened 95% instead of 75%, the choke cable had to be adjusted so that it fully opened and closed-it was almost a 1/4 on all the time,it was then we adjusted the mixture and idle speed and the car runs really well- except for the slight misfire at idle,the only things we can think of is condensor and generator brushes worn or corroded.

With yours check that the vacuum advance is working like it should,that the breather from the rocker cover to SU seals properly and and if your running the standard air filter make sure its clean or change it for a K&N.

My 2c
Author:  sgc [ Thu May 07, 2009 11:23 pm ]
Post subject: 
74snail wrote:
With yours check that the vacuum advance is working like it should,that the breather from the rocker cover to SU seals properly and and if your running the standard air filter make sure its clean or change it for a K&N.

My 2c


Thanks for the contribution :) There's no vac advance on this one, it's a 43D4 dizzy. I'll have a closer look at the breather but it all looked sound when I put it back together.

The air filter appears to be clean, but maybe I'll take it out for a run without it just to eliminate this one...

[edit]

Checked all the breathers, they're intact & secure.

Closed the plug gaps to .025" for the sake of the exercise, and took it for a run without the air filter (which looks brand new and clean enough to eat off). If anything it's worse :evil:
Author:  steve d [ Fri May 08, 2009 12:33 am ]
Post subject: 
It could be your head gasket between 2 and 3. When pressed hard it may be letting the fuel and air into the next chamber.
Author:  david rosenthal [ Fri May 08, 2009 3:05 am ]
Post subject: 
With the idle mix set leaner or with a lighter oil in the dashpot the symptoms set in lower in the rev range, but are lessened with application of the choke. This I think points towards there being enough fuel in the float bowl.. if there wasn't, asking for more fuel with the choke wouldn't work.[/quote]

fuel starvation problem. a few things to check
1 run the eng at idle ,shut off and then carefully remove the float bowl lid. measure from the top edge to fuel level . it should be about 7/8"
2 see how far down in the carby body the jet is[remove piston] and then slowly tip a little fuel into the bowl. see if it comes out of the jet immediately. The needle top shoulder should be level with the base of the piston.
3 check that there is no blockage between the fuel bowl and the jet. some times the little rubber sealing washer has not allowed the plastic tube to push thru and partially blocks the tube.
4 check that there is no little bits of rubbish in the inlet chamber to the float needle
5when you fitted the new float did you check the correct setting. invert the float and top assembly. measure from the center of the float to the lid. This should be 1/8" to 3/16". Also check that the float needle is free with no crap in the needle seat

when you use the choke all this does is to lower the jet top below the fuel level and allows fuel to run into carby. by using thick oil in the dash pot it is now the air control to the eng instead of the butterfly. The spring should be a RED
With the eng idleing if you lower the jet,ie richen the fuel does the colour tune change colour. you should be able to adjust the mixture by a couple of turns and it should be black
Author:  BALLISTIC [ Fri May 08, 2009 3:39 am ]
Post subject: 
74snail wrote:
With mine we reset the tappets,rechecked the spark plug gaps, checked the points,advanced the timing a little, we then checked the accelerator and choke cables-the accelerator cable needed to be adjusted so that it opened 95% instead of 75%, the choke cable had to be adjusted so that it fully opened and closed-it was almost a 1/4 on all the time,it was then we adjusted the mixture and idle speed and the car runs really well- except for the slight misfire at idle,the only things we can think of is condensor and generator brushes worn or corroded.

With yours check that the vacuum advance is working like it should,that the breather from the rocker cover to SU seals properly and and if your running the standard air filter make sure its clean or change it for a K&N.

My 2c


Yours is in the head.....valve not sealing or recessed in the seat, etc
Author:  BALLISTIC [ Fri May 08, 2009 3:48 am ]
Post subject: 
sgc.......whats the exhaust like, flowing free no blockages?
whats the resistance in the leads? maybe one is breaking down under heavy load?
If its a mechanical fuel pump maybe its not able to keep the pressure up at heavier engine speeds?
Does it go ok when you pull the choke out a little when it would normally misfire?
Author:  66S [ Fri May 08, 2009 6:42 am ]
Post subject: 
Hmm, sound like a fuel starvation problem to me. Have you looked at the fuel pump?

Al
Author:  guna mk1 [ Fri May 08, 2009 7:39 am ]
Post subject: 
What about the rotor button? They can develope leakage and cause a missfire under load. I have experienced this problem.
Author:  DOZ [ Fri May 08, 2009 7:51 am ]
Post subject: 
I would have said something to do with fuel flow - the majority of others I've seen has been with the filter being blocked, 2nd being dodgy fuel pump (either elec or mech).

Maybe take the hose off the carby and check the fuel flow (time how long it takes to fill a certain volume)?

Daniel
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