BMW K100 throttle lagg and exhaust 4 not heating up

[Dennisvp]

Dear all,

For the past few years, I have been working on restoring a 1987 BMW K100 LT.

I feel like I have ruled out many potential issues, but I still can’t get it running properly and hope to get the final push in the right direction. The engine starts (without choke) immediately and idles just above 1,000 RPM. The problem arises when giving throttle. When I open the throttle, the engine has a delay in increasing RPMs and then suddenly jumps to 3,000 RPM. The issue lies in the delay when accelerating, along with exhaust #4 not getting hot.

- I haven’t disassembled the engine, but I have measured the compression and valve clearances, which are within spec.
- I secured the rubber manifolds under the throttle bodies with a small amount of liquid gasket, eliminating any air leaks.
- I installed 4 new injectors from Kpartsholland.
- New thermostat and thermometer.
- Custom air filter (DNA open air pod filter).
- The spark plugs are firing well. After testing the engine, the spark plugs are wet with fuel.
- Exhausts 1, 2, and 3 get hot (200°C), but exhaust 4 does not heat up and only reaches 80°C at idle. If the air filter is blocked, it can rise to 120°C.
- TPS was set to click when initiating or closing the throttle.
- Vacuumhose replaced by new from Tils.de

Any tips are welcome! If any suggested advice has already been tried, I will add it to my post.


Best regards,
Dennis

[daveson]

One thing to check is the vacuum hose from the fuel pressure regulator to throttle body four. Remove the hose from throttle body four to check if it's dry, it should be or else the fuel pressure regulator is leaking. Or the hose might be split or disconnected.

[Dennisvp]

Thank you Daveson, I have replaced the vacuum hose for a new one. Will check if it is dry.

[The Mighty Gryphon]

The Jetronic system will have a small amount of lag built in to the system because of the flap in the air flow meter.  That flap acts as an obstruction that is moved out of the way by the air being sucked into the cylinders. 

As far as cylinder #4, are you sure the spark plug is firing and the injector is being pulsed?  A bad spark plug will fire at idle, but as the throttle is opened and the compression in the cylinder increases it can fail to spark.  You could try swapping the spark plug from #4 with #1 and see if the problem moves with the plug.

You could also try the same thing with the injector.

[daveson]

Another thing, the spark plug lead to cylinder four might be bad.

[Dennisvp]

As far as cylinder #4, are you sure the spark plug is firing and the injector is being pulsed?  A bad spark plug will fire at idle, but as the throttle is opened and the compression in the cylinder increases it can fail to spark.  You could try swapping the spark plug from #4 with #1 and see if the problem moves with the plug.

Another thing, the spark plug lead to cylinder four might be bad.

Thank you Mighty Gryphon and Daveson

The spark plugs give good spark, only havent tested if it fires at the right moment.

I will try and swap the injectors and test the spark plug leads to see if that gives any results.

For now I have detached the throttle bodies again, doing another sync on them. As the they have been completely apart for sodablasting (yes they have been thoroughly cleaned and sprayed out afterwards) the factory settings are no longer there. I have synced them previously but might not have had the TB idle screw all the way out. Will do another resync. If anyone has a video or manual on this, it is highly appreciated as the manuals only describe how to set the air flow screws.

[Laitch]

A bad spark plug will fire at idle, but as the throttle is opened and the compression in the cylinder increases it can fail to spark.  You could try swapping the spark plug from #4 with #1 and see if the problem moves with the plug.
You could also try the same thing with the injector.

The spark plugs give good spark, only havent tested if it fires at the right moment.
I will try and swap the injectors and test the spark plug leads to see if that gives any results.

Swapping the plug would be quicker for elimination of that suspect right off the bat.

[The Mighty Gryphon]

It is harder for a plug to spark in the high pressure in the cylinder.  Checking them out of the engine doesn't tell you whether or not they will fire in the engine at Top Dead Center when the pressure is much higher.  Swapping with another plug will tell you if that is the problem.

If the problem follows the plug, the plug is bad.  If not, the wire or coil is suspect, or possibly the injector. 

[sooprvylyn]

Id say its likely a plug or a wire. If a coil was bad he'd probably have 2 cylinders with issues since there are only 2 coils. 

-Swap the plug.  if the issue follows the plug, its the plug.  If the issue stays on the cylinder, its probably the wire or injector.

-Swap injector.  if the problem follow the injector, its the injector.  If it stays on the cylinder after swapping both plug and injector its  a bad wire

it could also be possible that the intake on that cylinder has an issue since you've completely disassembled it for sandblasting. Should be pretty easy to check function tho and rule that out.

You've already mentioned that compression was good, so its almost certainly a bad plug, bad wire or bad injecctor.

[Dennisvp]

I have taken the throttle bodies off the bike to rebalance them. Found my mistake and balanced them the correct way. The bike now revs instantly and drives as it should.

Thank you for all the responses!

[Laitch]

Found my mistake and balanced them the correct way.

Share your mistake here, Dennis, so others will not make the same mistake.

[Dennisvp]

Share your mistake here, Dennis, so others will not make the same mistake.

The mistake was that the TB's werent synced properly. After resyncing them, they bike runs fine.

[Laitch]

The mistake was that the TB's werent synced properly. 

How did that happen?

[Dennisvp]

I synced them myself, but had not loosened the other TB's before setting them to #3.