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Admitting defeat - 71 twin port misfire
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richard1263
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PostPosted: Mon Apr 25, 2016 7:21 am    Post subject: Admitting defeat - 71 twin port misfire Reply with quote

.After rebuilding a 71 twin port to stock spec I just can’t get to the bottom of a misfire. the car idles fine and does not misfire when revered but as soon as I pull away the car splutters and back fires. I normally find any answers by reading previous posts on the samba but this has got me mystified. I have listed the items checked to save time but would really appreciate any help from you guys.
1) Fuel in. changed filter and checked pump.
2) Check mixture, idle, and jets and used two pict 31/30 carbs, one of which is new.
3) Checked sparks at plugs, fitted two new sets.
4) Checked leads, three sets one new and one of a working engine.
5) Changed rota cap x3 one from a working engine.
6) Changed condenser and points.
7) Swapped vacuum distributer for a 009.
Cool Used wd40/easy start and propane to check for air leeks, no increase in speed.
9) Set timing with lamp and strobe. 7.5 BTDC. Printed pulley.
10) CHECKED THE LEADS GO FROM THE DISTRIBUTER TO THE RIGHT CYLINDERS, 1,4,3,2
11) Checked the valve gaps.
12) Replaced fuel tank and blown through the line.
13) Checked oil pressure 20 PSI.
14) Checked compression on cylinders 1, 128, 2, 128, 3,121, 4, 128 PSI
15) removed large dent from the rear panel, not related just got hacked of one day…………………
Cheers.
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PostPosted: Mon Apr 25, 2016 7:48 am    Post subject: Re: Admitting defeate Reply with quote

needs new set of eyes to look at it.I would first put a set of bosche platinum plugs in it.
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PostPosted: Mon Apr 25, 2016 8:19 am    Post subject: Re: Admitting defeate Reply with quote

7) Swapped vacuum distributer for a 009.

I'd say this is your problem. Oh and you said two 30/31s? dual carb?
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richard1263
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PostPosted: Mon Apr 25, 2016 8:31 am    Post subject: Re: Admitting defeate Reply with quote

swapped to 009 just to see if this made a difference as this worked fine on my other engine.
single carbs, first came with the engine that I rebuilt and the second is a new one.
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PostPosted: Mon Apr 25, 2016 8:48 am    Post subject: Re: Admitting defeate Reply with quote

Stumbles coming off idle under load?
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PostPosted: Mon Apr 25, 2016 8:50 am    Post subject: Re: Admitting defeate Reply with quote

Hi,
Just curious, you gave us your idle advance; what is it reading at 2000 RPM? Also, any exhaust leaks? I assume it's backfiring through the exhaust and not the intake?
Thanks--
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richard1263
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PostPosted: Mon Apr 25, 2016 9:05 am    Post subject: Re: Admitting defeate Reply with quote

Haven't checked at 2000 RPM but have revered hard and watched the pulley marks to 31 degrees. not sure were the back fires coming from as it only does it under load., had a very small blow from the muffler but that is it.
The noise is more of a pop than a ping..... now that's tec talk for you... going to put her back on the ramp this evening to check the exhaust for leaks though.
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PostPosted: Mon Apr 25, 2016 10:14 am    Post subject: Re: Admitting defeate Reply with quote

Are these new carbs or old ones? Are you sure the jet paths, particularly the idle jet path is not partially blocked? What about the accelerator pump nozzle? Is it flowing plenty of fuel? Stick you head over the top of it without the air cleaner on and pull the throttle. Does the stream go straight down to the butterfly or hit the wall? If it hits the wall then use a screw driver to move it over.

Lastly the main jet may be too small especially if they are new carbs. Buy these: http://www.amazon.com/SE-Drill-Mini-Sizes-82616MD/...detailpage
and this: http://www.amazon.com/General-Tools-93-Adjustable-...detailpage

Use the drill bits to drill to the next larger drill bit size. Slip the one in the main jet that fits and use the next larger size. It's possible the idle jet might need drilled up too. This depends on a number of factors including new or used carb, compression and some other stuff. I hope this helps.
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PostPosted: Mon Apr 25, 2016 11:01 am    Post subject: Re: Admitting defeate Reply with quote

One new carb repro suitable for 71 1300cc and one old. Both show identcle symptoms when pulling away. Pump moving fuel ok when tube is removed from carb, haven't checked inside the carb just swapped it over.dismised the main jet for the same reason unlikely to be two wrong size jets. But will check both anyway. I've noticed after removing the new carb that is is very black inside, not to get to distracted ive assumed this is from running it to rich while attempting to solve the problem??? Could this be a clue?
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PostPosted: Mon Apr 25, 2016 6:32 pm    Post subject: Re: Admitting defeate Reply with quote

Richard1263
Repost this in the performance forum. There are alot of very smart guys there that can and will help you. Make sure to listen to what they have to say and do things in the order that they tell you. They will have you on the road in no time.
Good Luck.
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PostPosted: Mon Apr 25, 2016 10:45 pm    Post subject: Re: Admitting defeate Reply with quote

Moved...
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PostPosted: Mon Apr 25, 2016 11:29 pm    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

Thanks for reposting and correcting the spelling.
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PostPosted: Tue Apr 26, 2016 1:18 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

Need more clues.
How old is the gas in the gas tank?

Is it worse hot or cold? rich/lean
worse with choke on/off/dissconnected/propped open?
Is the airfitler odd? Tried drving with it off?
is the muffler suspect?


Is there any clues it is carburetor related at all?

Have you tried alternate ignition timing? does it do better advanced or retarded? Tried swapping coils? Could it be a wiring problem? Loose battery connection, ground straps, wobbly connectors, worn out ignition switch?
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PostPosted: Tue Apr 26, 2016 4:33 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

are them 'leads'/spark plug wires the cool colored ones?
I found them to cause the same issues you are having Rolling Eyes
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PostPosted: Tue Apr 26, 2016 6:27 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

Are you sure 100% that you have you spark plug wires on the cap in the correct orientation?

Find TDC on the compression stroke. Find out where the rotor is pointing at on the distributor. Put #1 cylinder wire on the cap under where the rotor is pointing. Pay no attention to the notch on the distributor at this point unless the rotor is pointing at it on TDC compression stroke.

the firing order is 1-4-3-2 with the rotor moving clockwise inside the distributor cap.
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PostPosted: Tue Apr 26, 2016 6:32 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

Did you open up the carb and take out the jets and blow them out with carb cleaner/air-it's only like a 20 min job.

Maybe post a photo of your engine as well-visuals sometimes help.
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PostPosted: Tue Apr 26, 2016 7:08 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

The exhaust could be x% plugged. I have had that happen a couple times before. It will idle all day long but as soon as you try to drive, it sputters.
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PostPosted: Tue Apr 26, 2016 7:46 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

Wow more questions than answers here guys.
have tried nearly all the things you all asked, coil, leads, jets ect.
But can you elaborate more with regards to.
Earthing, connections, in particular the areas I should investigate first.
Muffler x% ???.
Haven't tried running with pancake filter off but will do in the next few days.
I new the question of lead order would crop up again, I guess this is the most common mistake, but they are defo in the right order.
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PostPosted: Tue Apr 26, 2016 7:49 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

I think what your saying is you have used 2 different 30mm carbs at different times and still have what you call a misfire that is probably more like the vehicle falls on its face at take off. If I were you, I would find a good 34 pict 3 carb and drill the idle jet to .65mm they came with the idle jet at .55mm if I recall correctly....if not someone correct me. Also, the 34 adjusts idle speed differently than a 30. The butterfly on the 34 should be completely closed at idle as adjusted by the screw on the arm. the idle speed (large screw, left side) and the small screw, adjust the idle speed and mixture, respectively.

The ideal setup would be the 34 carb with a dual vacuum advance distributor.
I have had cars that would hesitate and fall on their faces just as you describe with the same specs as your motor. The repair was to put a dual vac distributor on the car. The vacuum lines, of course, need to be installed correctly. Here is what I think happens using the dual advance distributor. The larger green vacuum line goes to the rear vac chamber of the distributor and has vacuum when the throttle is closed (idling) At the hit of the throttle, vacuum drops and the distributor immediately advances the timing. Using a 009, the timing advance doesn't come in till much later, and by then it to late.....if you can't find a good dual advance distributor (check both sides of the can) use the 009 and drill the idle jet. You will need a pin vise and jet drills for that operation.
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PostPosted: Tue Apr 26, 2016 8:16 am    Post subject: Re: Admitting defeat - 71 twin port misfire Reply with quote

ok I get most of this post.
think I have spare .65 jets could I just install into the pict 30/31 carb?
don't have a dual vacuum distributer and not seen any sold in the uk.
I have tried 009 and single advance,. which is new.
getting some good feed back from you all ,plenty to check over the coming days, cant help thinking I need to draw up a plan of action so as not cause more problems along the way.
still intrigued about the comments regarding the exhaust/ muffler and earthing.
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