rough iddle issue again

deQuincey

deQuincey

Quousque tandem...?
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hi,

after the terrific tip of mr porsche about the tiny hole for the iddle air was completed, my iddle was somehow tuned a bit, i mentioned that the iddle was a little up (around 1000 rpm´s) but i will nt complain on this now

anyway i have to say that the iddle is not smooth, and the engine vibrates a bit

this has been my test-check then:

1-i have two carbs, ok
2-each one feeds one admission tube, that splits into three, one for each cylinder
3-noting the engine iddle is rough, i decide to check the sparks in each cylinder, so i unplug the spark plugs one by one, and the result is the following:

when i unplug the central cylinder of each carb (this is cyl No. 2, and 5), the engine dies clearly and quickly

but when i unplug any other cylinder, the iddle harldy changes, apparently you can tell no big difference

my thoughts:

the ammount of fuel that is being fed by the carbs might be not enough for the three cylinders, and thus the central cylinder is the one taking the right ammount of fuel, meanwhile the other two cylinders are getting not enough

but, if i increase the opening of the iddle screw in order to take more fuel from the reservoir in the iddle operation, then then revs. will go up for sure !

any ideas ?
can the mixture screw help this ?
what about butterflies?

thank you and regards
 
Lean fire

Your middle cylinders are running too lean.

This carb condition can be alterted by:

1.Slightly closing both primary butterflies with it's adjusting screw.( Less air.)
2. Slightly opening the idle screws. ( More fuel.)

Preferably, use a combination of both in moderation until you get to a smooth running condition. These adjustments are tiny as in degrees or minutes and not 1/4 turns once you get the middle firing. And wait about 30 seconds after each adjustment as the intake charge takes a moment to change inside the manifold.

It can be frustrating sometimes at this point because what seems to be the solution is just the opposite. I also like to use new or clean plugs and pull those two to do a visual on them.
 
I never tried pulling plug wires to see what would happen so I can't comment on that. But, I probably spent more time trying to track down rough idle problems on my '74 CS with original Zeniths, then Weber 32/36s, than anything else. Electronic ignition, NGK plugs, carb rebuilds, new jets, nothing worked. Then my engine developed a persistent mis-fire (which disappeared when the engine heated up) and so I figured I had a head gasket problem (even though cylinder pressure checks were OK.) I pulled the head and discovered small coolant leaks in 1 and 2 cylinders. Had the head planed, installed a new gasket and voila, no more rough idle. The moral of the story is that rough idle problems can be hard to diagnose (duh...) Good luck with this.
 
any additional symptom can assess this ?

bill said:
Then my engine developed a persistent mis-fire (which disappeared when the engine heated up) and so I figured I had a head gasket problem (even though cylinder pressure checks were OK.) I pulled the head and discovered small coolant leaks in 1 and 2 cylinders. Had the head planed, installed a new gasket and voila, no more rough idle. The moral of the story is that rough idle problems can be hard to diagnose (duh...) Good luck with this.
Click to expand...

my cylinder pressure checks seem to be ok (all cyl between 10,5 and 12 bars)

my head has been recently done with apropriate head gasket and new bolts (i did not do it by myself and i am not very confident on this)

nonetheless even if i have renewed the coolant several times (including the draining engine bolt), (i will say that the coolant-renewing operation has been made 3 times in the period of 2 years), my coolant is always dirty (it includes dark colour (in original condition is green), and "sand" deposits in the botom of the coolant reservoir when the engine has been resting for a while), well i say "sand", but i have not touched it, when i try to touch it melts with the coolant

explanation of some many times renewal: 3 times are a lot, but this is the reason:
1st. renew of all the cooling system, including, radiator, water pump, hoses, thermostat, coolant reservoir,...
2nd. head issue,
3rd. broken camshaft, it was done through the front, and thus the radiator was taken away for the operation
 
step by step understanding

61porsche said:
Your middle cylinders are running too lean.
the thesaurus says that "lean" is "poor mixture", right ?

This carb condition can be alterted by:

1.Slightly closing both primary butterflies with it's adjusting screw.( Less air.)
i understand that this is the screw that stands alone out of the carbs, in between the main body of the carb and the heated cold start choke, right ? see green arrowed in picture

2. Slightly opening the idle screws. ( More fuel.)

i know one iddle screw, is it the only one ?, i have been reading some literature on the zeniths, and i am finding other mixture fixing screws but they seem to belong to different carb models, my iddle screws are red arrowed in the pic

Preferably, use a combination of both in moderation until you get to a smooth running condition. These adjustments are tiny as in degrees or minutes and not 1/4 turns once you get the middle firing. And wait about 30 seconds after each adjustment as the intake charge takes a moment to change inside the manifold.

It can be frustrating sometimes at this point because what seems to be the solution is just the opposite. I also like to use new or clean plugs and pull those two to do a visual on them.
Click to expand...


question: are there any other adjustment screws over the carbs ?

i mean: provided the automatic cold start choke works reasonably well, i assume that part will remain untouched

regards and thanks

karbus.jpg
 
When the head was pulled, did you have it pressure tested for cracks? (I was lucky...my original '74 head passed that test.) A cracked head might explain why your coolant is dirty.
 
Re: the coolant leak, maybe the head bolts (one or more) are not sufficiently torqued (you mentioned you are not confident about the head work you had done...) When I did my head, I used the torquing procedure specified by the gasket manufacturer (i.e. torque to whatever ft/lb., then a two-stage final torque by degrees. The old way described in the manual calls for torquing in stages with increasing ft/lbs each stage.) Either way should work if done correctly. Put a torque wrench on the bolts and make sure they are all tight.
 
Understanding...

Yes, the idle is poor for the number 2 and 5 cylinder. In this case, the mixture has more air that needed.

The green arrows point to the screws that control the opening of the primary butterflies. To reduce the amount of air, turn counter clockwise in small increments. It is important to note to make sure that the carburator linkage is not hanging up ( keeping the butterflies open) by pressing down on the linkage. If this condition is observed, then adjust the linkage slightly smaller in length.

The red arrow points to the idle screws. Counter clockwise opens the passage for more fuel.

The choke- only effects starting and perhaps 3-5 minutes during warm up. There is a daiphram under the triangle shaped part which if leaking can effect idle quality as vacumn is present full time there. That part is thin rubber. If it is hard, like plastic in appearance or light touch; it has seen better days. Don't try to remove it until a new part is in hand and can be done without carb removal. The single most thing effecting easy startup is to make sure the choke flaps move together closing and opening the same amount and timing. By far and away, the rod is just out of it's range. It must be held upward, flap closed, then the setscrew tightened. ( I realize you didn't ask; so this is for all the owners who complain about hard starting with carbs). The final adjustment here is the small screw on top of the triangle carb diaphram. This controls the amount of choke opening when the engine starts. Again , sometimes the best an owner can do is to make sure that all the adjustments are visually first about the same for both carbs.

If I didn't answer everything, please ask again.
 
yes understanding !

61porsche said:
Yes, the idle is poor for the number 2 and 5 cylinder. In this case, the mixture has more air that needed.

this is my first understanding problem, in the spark check, the cyl 2, and 5 were the only ones that, when their spark was supressed the engine died. so, can it be assumed that those two cylinders were the only ones receiving the apropriate mixture ratio ? and thus the rest are receiving lean or poor mixture ?

The green arrows point to the screws that control the opening of the primary butterflies. To reduce the amount of air, turn counter clockwise in small increments. It is important to note to make sure that the carburator linkage is not hanging up ( keeping the butterflies open) by pressing down on the linkage. If this condition is observed, then adjust the linkage slightly smaller in length.

The red arrow points to the idle screws. Counter clockwise opens the passage for more fuel.

ok, now discuss this part:

i see that a richer mixture is needed for the iddle fuction, in order to permit adequate mixture, so: less air or more fuel, ok !

but anyway: richer mixture will mean increase revolutions ! right ?

so, considering that now i have 1000 rpm´s, how to go back to 900 ? or even to 1000 ?

.
Click to expand...


thank you !
 
Understanding mixture

Once you have it running smoothly as possible, go back and try smaller still adjustments with just the idle screws. Think in terms of minutes of adjustments than 1/4 turns. And patiently wait a minute or longer for the mixture path to clear for the new adjustment.


You asked about coolant. I would say it is a perception as many owners report the same. Wether the overflow tank is clear and reflecting the darker engine, etc. is what most find. Some report the rubber components emit particles. Because of the normal heat cycle in the engine, I would expect some small scale. Coolant leaks in the combustion chamber usually show up by very cleaner than normal spark plugs and cylinders.

There is a chemical test kit to check coolant and oil available at most auto parts stores for a nominal amount. Some work by visual idicators and others are samples sent off for microscopic analysis.

I would say that if you're not losing oil, no visible oil sheen in the overflow container, not to worry. If you do, buy a test kit.
 
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Idle check

Assuming your carbs are the same as here in Europe you can check them like this.

If you get the engine warm & then remove the wire from the idle solenoid (tubular thing about 40mm long & 20mm diameter at bottom of the carbs...) of one carb. If the engine idle gets slower then all is well. Check on both carbs. If however removing either wire has no effect then there is a problem with the idle system. Usually this is caused by the lower gasket block which is just above the butterflies. If you have the original blocks still fitted they are bakolite with a paper gasket glued on either side. The air for the idle system passes through a 3-4mm hole in this block and eventually the vacuum sucks the paper gasket accross the hole partly blocking it. This was a constant problem and BMW introduced a modified block with a rubber `O` ring in place of the paper gasket. End of problem.
It is however a pain to change them as you need to completeley remove the carb. Obviously you should check the basics before assuming this is the problem.

alex
 
i will try this

61porsche said:
about the coolant issue, i have no oil on the cooling reservoir
and also not milkshake on the oil

but i ought to consider checking the torque on the head bolts !
what was the process ? it was supposed to be done with the engine warm, and then i think is a 70 N.m final torque, isn´t it ?

regards
Click to expand...
 
the second part of understanding

61porsche said:
The choke- only effects starting and perhaps 3-5 minutes during warm up. There is a daiphram under the triangle shaped part which if leaking can effect idle quality as vacumn is present full time there. That part is thin rubber. If it is hard, like plastic in appearance or light touch; it has seen better days. Don't try to remove it until a new part is in hand and can be done without carb removal. these diaphragms were renewed some months ago when renewing the carbs, so i might assume they are ok, but as you said, i can check them easily, so probably i will do

sorry i don´t understand the following:
The single most thing effecting easy startup is to make sure the choke flaps move together closing and opening the same amount and timing.
first question; is the choke flap the butterfly located on top of the carb ?

By far and away, the rod is just out of it's range. sorry don´t understand this


It must be held upward, flap closed, then the setscrew tightened. ( I realize you didn't ask; so this is for all the owners who complain about hard starting with carbs). The final adjustment here is the small screw on top of the triangle carb diaphram. This controls the amount of choke opening when the engine starts. Again , sometimes the best an owner can do is to make sure that all the adjustments are visually first about the same for both carbs.

If I didn't answer everything, please ask again.
Click to expand...
.
 
Choke flap- yes, the aluminum butterfly on top.

Choke rod- the metal rod connected to above part.

The procedure requires a little dexterity:

Loosen the set screw on the choke rod linkage above. Engine cold. WHile pulling up on the metal rod ( middle and index finger), hold the choke flap closed completely ( thumb) , and tighten the setscrew ( free hand). Repeat for second carb. Both are now equal.

The choke diaphram has a limited motion or range and this procedure sets the range.

Mine starts first or second time always.
 
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ok let´s see

61porsche said:
Choke flap- yes, the aluminum butterfly on top.

Choke rod- the metal rod connected to above part.

The procedure requires a little dexterity:

Loosen the set screw on the choke rod linkage above. this is an allen (2,5) tiny screw, isn´t it ? (see red arrow in picture) Engine cold. WHile pulling up on the metal rod ( middle and index finger), hold the ckoke flap closed completely completely or interfere with a 2,5mm cilinder or drill ?
( thumb) , and tighten the setscrew ( free hand). Repeat for second carb. Both are now equal.

The choke diaphram has a limited motion or range and this procedure sets the range.

Mine starts first or second time always.
Click to expand...

carbchoke01118.jpg
 
Yes. Allen screw.

Closed. All the way for this procedure.


P.S. You are missing a small circlip on the rod under the allen screw and linkage. It aids in keeping the setting.
 
Last edited:
61porsche said:
Yes. Allen screw.

Closed. All the way for this procedure.


P.S. You are missing a small circlip on the rod under the allen screw and linkage. It aids in keeping the setting.
Click to expand...

yes you are right, i am missing that circlip,

ok, now i have the knowledge and the tools, but i do not have the time, so this weekend will be test time ! wish me good luck !

thanks
 
deQ, PM me with your email address and I will send you the various carb manuals in PDF which outlines all of this so you have something to read before now and Saturday...:mrgreen:

deQuincey said:
yes you are right, i am missing that circlip,

ok, now i have the knowledge and the tools, but i do not have the time, so this weekend will be test time ! wish me good luck !

thanks
Click to expand...
 
Head Bolt Torque

The head gasket I used was from Elring. The torque instructions are:
1st. step: tighten to 60+/-2Nm
Wait 15 mins.
2nd step:torque angle 33+/-3 degrees
warm up engine 25 mins.
3rd step:torque angle 35+/-5 degrees

I followed this procedure and haven't had any problems after 1000 miles of driving (so far...) Retightening is not necessary with the Elring gasket.
 
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