Misfire with GP2 cam???

[JeffS]

I recently had a LONG discussion with a TR6 mechanic who was diagnosing a misfire ever since installing a GP2 cam from Richard Good. To make a very long story short, we discovered that the issue came down to distributor phasing. Essentially, there's a shift in where the distributor needs to trigger the coil, and its retarded about 6-8 degrees from stock. You can retard your timing to alleviate this, or another option is to use a Crane ignition system so you can adjust the phasing. Or, you can add an adjustment feature to your vacuum advance and use that feature to get the breaker plate repositioned to correct the phasing. The Pertronix actually shifts phasing int he same direction as the GP2 cam, so if you run BOTH, the Pertronix isn't bad, its just a phasing issue causing misfiring!!!

Essentially, the rotor is firing to the cap terminal very early in the engine cycle. When the timing advances with rpms (as a proper distributor should), the rotor is between plug wire contacts inside the distributor cap. You need to reorient the rotor so it fires at its leading edge, rather than at its back edge. This problem actually exists in MANY TR6s, but really gets exaggerated with this particular camshaft. You can actually look at a good used rotor and see where its positioned while firing. There will be black burn marks present on the forward edge, middle, or rear edge. You want to see the burn marks out toward the "point."

This also occurs in other 6 cylinder distributors like the E-types, BJ8, and MGC.

 

[Trick6]

I have the GP2 cam and not experiencing any misfire. Is this misfiring occurring on a particular cylinder? I feel confident to say the motor pulls strongly right through the RPM range with out any hesitation or laboring.

My dyno sheets are posted at this address. Note that I stopped the pull at 5200 RPM.
https://www.6-pack.org/spaw/dyno/AlHP.pdf

I do wish I had installed the cam bearings when the motor was rebuilt. The cam bearings would have given me much more flexibility, allowing me to go to a bigger cam.

My motor prep profile is at the link below.

 

[JeffS]

This doesn't appear to be the case with ALL the cams, but it does happen. If you have an early distirbutor with a dual vac, that allows adjustment of the phasing, so its not an issue. The later retard-only distributors are the ones with issues. If you use points, its typically not a problem. If you add a Pertronix or an MSD box, then you're making the problem worse and it becomes an issue. Normally you can go back to points or the Crane and the issue is resolved, at least enough to smooth out the car. Retarding the timing can also help. I like to see them set at 10-14 degrees BTDC at idle (anywhere below 1000 rpms) with the vacuum retard disconnected.

 

[LastDeadLast]

Jeff,

Is the problem with the cam profile or the dizzy? For example, I'm running a Mallory uni-lite and haven't experienced any misfires.

-s

 

[Guest]

Jeff,

Is the problem with the cam profile or the dizzy? For example, I'm running a Mallory uni-lite and haven't experienced any misfires.

-s

Shannon,

I thought you just went through a long post at the end of July about misfiring at all RPM's with this GP2 cam and the Mallory uni-lite?

Turned out to be just hot carburetors?


.

 

[TheSearcherMan]

I had the GP2, and an APT, and now have the CP cam. I have never heard of anything like this. I suggest someone installed your cam wrong, and you get another mechanic. I have had no problems in this respect with any of the cams. Hardness seems to be the problem, and getting the lifters to all spin.

 

[LastDeadLast]

Shannon,

I thought you just went through a long post at the end of July about misfiring at all RPM's with this GP2 cam and the Mallory uni-lite?

Turned out to be just hot carburetors?


.

Stirke,

Not really sure what the problem was, but it definitely wasn't the dizzy; One the first things I did was test the dizzy as per Mallory's instructions and left it alone after it came out OK....

What I had was much worse than a misfire.. I couldn't even keep it idling. It might have been heat, a fuel pump issue or water in the tank.. I tried so many things, but at the end of the day, I slapped it back together and it worked. I've driven almost 500 miles with no issues. This is before I installed the new heat shield.

My theory is.. at the time I was playing with lean the mixture out a bit to get more power... this caused the engine to run a little hotter which then boiled the fuel in the carbs.. When I put it back together, I richened the mixture a bit and the carbs stayed much cooler. Now that the heat shield is on, I'm gonna start leaning it out again.

 

[crj7driver]

My Triumph mechanic told me about the same thing, he ended up re-indexing where the points opened. This was on a completely stock motor. It made a huge difference in smoothness and power not to mention allowed me to pass emissions.

He did say that a large number of Triumphs left the factory this way and no amount of playing with the time will truly fix the problem.

 

[JeffS]

I had the GP2, and an APT, and now have the CP cam. I have never heard of anything like this. I suggest someone installed your cam wrong, and you get another mechanic. I have had no problems in this respect with any of the cams. Hardness seems to be the problem, and getting the lifters to all spin.

Actually, this cam was installed by a shop that I rebuilt the distributor for, and the shop owner did the rebuild and cam install - properly. In fact, he spent some serious time on the phone with Richard to help figure out the problem, which is when he figured out the lobe separation on this cam is really 112 degrees - not as specified. He followed Richard's recommended setting to the letter and still had a massive misfire above 2500 rpms. Retarding the timing made the misfire go away. That tells me that either the cam was ground wrong, or the gear for the distributor drive had been shifted? Either way a simple (well not so simple) rephasing of the distirbutor WILL make the car run better!!!

 

[Brosky]

Glad to hear that the fix is simple.

No problems with either points or Pertronix with my TH5 cam from BPNW. But it's not quite as radical as the GP2. Of course, Jeff has done both of my distributors, so I know that I'm starting out with a great ignition system.

 

[TheSearcherMan]

I don't see why you can't rotate the distributor and compensate for any variations in the cam gear. Also, as for grinding the cam, I think they are cast very close to the right profile, and you don't grind that much off. That is why there are so many problems, only one blank is made, and they are being ground to alot of different profiles, and the hardness is ground thru. Triumph, cast the part number in each cam blank, so, this would indicate, each different cam was cast for that profile, not one cam for many profiles. And, Richard is very sharp, and you should be able to check the centerline without removing the cam. The centerline for several of these is 105 degress, or close to that, not sure about the GP2. However the APT was 104, and the CP is 105. The car will run if it is off a few degrees, but the power, and powercurve will change. These problems are impossible to fix online. However, all the things you mention, you should be able to check with a degree wheel, if you get the proper info from Richard. I don't agree with you Brosky, the TH5 is more cam than a GP2, as it should have alot more power, at least according to the people in the UK. The TH5 is basically a cam profile from some saloon car with carbs. That is why it is supposed to put out more HP than a CP. I am being told that Lucas designed the CP cam to work well with their PI. I think the profile versus lift is not designed to work nearly as well with a carb. The TH5 has the CP profile on the exhaust lobes, and higher lift on the intake, that is my understanding.

 

[BryanC]

I guess there are two messages here. One is about the accuracy of the GP2....ok.

The other is about phasing. I don't understand what is meant by 'phasing.' Like Searcherman, I don't know what is being done other than rotating the dist. to change the timing. So, what is 'phasing' and how does one do it?

Thanks,

Bryan

 

[DNK]

My understanding from this thread is where each plug fires in relationship to the rotor. Leading edge to trailing edge. Never thought about it before. It probably is pretty relative.

 

[TheSearcherMan]

No,there are three messages here. This thread shows one of the things that is wrong with America today. Where the plug fires, we call that setting the timing here in Virginia. If you have a mis fire above 2500 rpm, and not at 1000, then you have an ignition problem, not a lobe problem. If you try to set the timing at 14 deg. at idle, its not going to idle very good. There is nothing wrong with the lobe placement on the GP2. LIke I said, get another mechanic. This phasing deal, this is BS. Don't worry, this is all I have to say about this subject.

 

[JeffS]

Picture the points location in the distributor. When the points open to a certain gap, spark will jump across the gap and fire the cylinder the rotor is at (surely an abbreviated version so don't go there - that's not the point.) If you change the relationship of the breaker plate versus the distributor housing, you change the part of the rotor that is AT the cap terminal WITHOUT changing the timing. If your phasing is set up so that the rear edge of the rotor is firing to the cap terminal, and your timing advances, then you'll be increasing the gap between the rotor and cap terminal if not risking a spark jumping to the NEXT cap terminal.

Saying "phasing is BS" is like saying you can install a Crane EI kit and it doesn't matter where the module gets mounted. IT DOES MATTER! The early dual vac distributors you actually adjust the phasing when you adjust the timing with the avcuum retard module. With the later retard-only distributors, phasing is fixed by the length of the spring-thing on the vac unit. If you install an adjustable vacuum unit, it will allow you to change the phasing.

I'm really not sure how the GP2 cam is different from others and why phasing becomes an issue using this cam, but it does. I'm not cam expert, but I do rebuild thousands of these distributors and am absolutely positive this issue exists!!!

How to check YOUR phasing:
Check your timng with a timing light at idle, with the vacuum line disconnected from the vacuum advance. Shut the engine off and line up the timing marks exactly like you saw them at idle. Now check to see where that rotor is positioned in relationship to the cap terminal its firing on.

 

[BryanC]

Jeff,

Thanks for the description of phasing. Is there one 'phasing' that is always better than another? If the ignition event (spark) happens at the same crank angle, what impact does the phasing have? That is to say, what difference does it make if the rotor sparked from the leading edge or the trailing edge if the spark in the cylinder happens at the same crank angle? Is the issue that if you are at one extreme at idle, then with advance the rotor is too far from the post to spark (or sparks to the wrong cylinder)? Still, that sounds like a distributor problem instead of a cam problem.

It seems to me that the relationship between the plate, points, rotor and cap is all internal to a given dist. How does the cam impact any of that? I can see that you might have to move the dist. relative to the block to get the same timing when you go to a different cam but I'm still not getting how the cam interacts with the phasing.

Thanks,

Bryan

 

[poolboy]

Jeff, for a moment let's forget about the cam; you've already made it clear that's not your forte'. However, everyone knows you are the man for distributors IMO.
What I am visualizing is that this "out of phase" is possible regardless of the cam. Am I correct? If I am, does a little too much advance at idle,can aggrivate the situation as speed and timing advance increases?
And, I for one appreciate your taking the time to inform of this trade secret of sorts. pb (a satisfied customer)but NFI.

 

[JeffS]

For some reason, phasing problems always seems to come up with the GP2. Why? I honestly haven't had the time to figure that out yet. I just know it exists. I have nothing against the cam. Its just that this cam always seems to be tied to the problem.

The problem lies in the rotor moving too far from the cap terminal as the distributor advances. Its likely a combination of problems, one of which is replacement vacuum retard units. The length of the arm which attaches to the breaker plate is the real key here. Many times its too short - shorter than the original - causing a phase shift. The location of the pin holding the vacuum unit in place and the depth of the distributor casting where the vacuum unit fits are all factors as well. Let's just say that Lucas distributors aren't exactly consistent from one unit to the next!

Now as far as changing this during a rebuild - well phasing is directly influenced by degreeing a cam. If the instructions for the GP2 include advancing or retarding the cam significantly from the factory cam position, then that DOES change the phase. If the centerline of the intake and/or exhaust lobes are shifted, then that changes the phase. My understanding is that this cam has a late exhaust centerline which would mean the timing should be slightly retarded from the factory setting for good exhaust scavenging, but if the cam gear is not ground retarded, that causes a phase shift. I'm working off of a lot of second hand cam info here, so I'm kind of hoping one of you will step in and relay more specific cam info to help me wrap my head around the cuase of the problem. Maybe its time to call Richard Good.

 

[swift6]

Jeff, do you know if this is specific to reground GP2's or new GP2's? I had a similar issue with my GP3. I could tell from marks on the rotor as well as marks on the cap that the phasing was off. I still have the timing spec sheet that Richard sent me with mg GP3, which was made on a reground Stanpart camshaft, if you need any info from it. I know its not a GP2 but if you need it for your database.

 

[JeffS]

I only recently put 2 and 2 together that this issue was common to the GP2s. I'll be keeping closer track of problems that I hear about and what cams they are used with in the future!!!