VFR750F-L (1990) - lumpy running / backfire

Hi fellow listers - I have some questions for you all:

My 1990 VFR is not running quite right - the symptoms are that on tickover
it sounds 'lumpy', it backfires sometimes when I decelerate, and fuel
consumption has decreased from around 45 to 35 mpg.

In order to replace a rusted-through exhaust, I recently had to take off
the front cylinder head, in order to remove a broken exhaust stud (it ended
up getting drilled out by a local machine shop). When I replaced the head,
I was not quite certain that I had got the valve timing gear teeth set
correctly... although I followed the instructions in the Haynes workshop
manual, it's not exactly clear where the timing marks need to be.

So, my first question is, if I had re-assembled the valve timing gear, say,
one or two teeth away from how it should be, would the engine even start /
run at all?

Secondly, if it's not the valve timing gear, what else might it be?
Unbalanced carbs? Dirty/stale fuel? Anything else??

All suggestions gratefully received!

Many thanks in advance...

Ian Larden, Portsmouth (UK)

(had - 1959 BSA C15 (black), 1966 BSA C15 (blue), 1970 BSA Starfire (
maroon), 1980 Honda CB250RSA (Red))
current ride - 1990 VFR750FL (Red)

Internet=ian_larden@xxxxxx

Home = +44(0)23 9226 6657
work = +44(0)23 9256 3925
Mobile = +44(0)771 880 2276

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On 21 Jun, 2006, at 18:10, Ian Larden wrote:

>
> Hi fellow listers - I have some questions for you all:
>
> My 1990 VFR is not running quite right - the symptoms are that on
> tickover
> it sounds 'lumpy', it backfires sometimes when I decelerate, and fuel
> consumption has decreased from around 45 to 35 mpg.
>
> In order to replace a rusted-through exhaust, I recently had to
> take off
> the front cylinder head, in order to remove a broken exhaust stud
> (it ended
> up getting drilled out by a local machine shop). When I replaced
> the head,
> I was not quite certain that I had got the valve timing gear teeth set
> correctly... although I followed the instructions in the Haynes
> workshop
> manual, it's not exactly clear where the timing marks need to be.
>
> So, my first question is, if I had re-assembled the valve timing
> gear, say,
> one or two teeth away from how it should be, would the engine even
> start /
> run at all?
>
I've no first-hand experience with these things, but the description
sounds about right for messed-up timing. fwiw, I _strongly_ recommend
owning the Honda shop manual.

Here's the instructions on replacing the cams from my Honda workshop
manual (90-96, USA printing) for comparison:

When only the front cylinder camshafts were removed:

Align the T1 mark on the starter clutch with the index mark on the
right crankcase cover.

Remove the rear cylinder head cover.

Make sure #1 cylinder is at TDC on the compression stroke. If it is
not, turn the crankshaft clockwise one full turn (360) and make sure
T1 aligns with the index mark on the right crankcase cover. Rotate
the crankshaft clockwise 1 1/4 turns (450) to align the T2 mark on
the starter clutch with the index mark on the right crankcase cover.

Apply molybdenum oil solution to the cam lobes.

Install the camshafts onto the front cylinder head so that the timing
_arrows_ on the camshaft gears are level with the cylinder head upper
edge and facing opposite each other. [picture points to the timing
arrows on the gears, and they are as far away from each other as they
can be.] NB: Honda appear to distinguish between timing _marks_ on
the rear camshafts and timing _arrows_ on the front.

INstall the camshaft holder dowel pins onto the rear [I'm hoping they
really mean front here] cylinder head. INstall the holders in the
correct positions, checking the identification marks on the holders
and the cylinder head. Tighten the holder bolts in the sequence as
shown. Torque 12N*m. Make sure the timing _arrows_ on the camshaft
gears are level with cylinder head upper edge and facing opposite
each other. [picture shows the cylinder head, with intake side on the
bottom, holder bolts are numbered (L-R, in four rows from top--
exhaust side-- to bottom--intake side--):4, 7, 6, 2; 8, 3, 1, 5; 5,
1, 3, 8; 2, 6, 7, 4;

Valve timing is correct when the timing _arrows_ are level with the
cylinder head upper edges, and the intake and exhaust timing _arrows_
face opposite each other.


> Secondly, if it's not the valve timing gear, what else might it be?
> Unbalanced carbs? Dirty/stale fuel? Anything else??
>
Unbalanced carbs can cause similar effects, but it typically doesn't
happen overnight. How suddenly did these symptoms come on?

later,
Johnj
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John, Ralph,

Many thanks for your responses.

To answer John's question - the symptoms started as soon as I got the
engine running and took the bike out for a test run, which happened
straight after fitting the new exhaust, which in turn necessitated removal
& replacement of the front cylinder head, for reasons described in my
previous post.

I think the bit of the instructions missing from the Haynes manual is
"Rotate the crankshaft clockwise 1 1/4 turns (450) to align the T2 mark on
the starter clutch with the index mark on the right crankcase cover" -
I don't remember doing this, and I'm pretty sure I followed the steps in
the Haynes manual.

Anyway, I'm sufficiently convinced by your words to take the front valve
timing gear off and re-position it.

My follow-up question now is this - in which direction should I rotate the
front timing gear - forwards or backwards one notch? Is it possible to
deduce this from the symptoms I described?

Many thanks for your help,

Ian Larden,
Portsmouth (UK)

(had - 1959 BSA C15 (black), 1966 BSA C15 (blue), 1970 BSA Starfire
(maroon),
1980 Honda CB250RSA (Red)) current ride - 1990 VFR750FL (Red)

Internet=ian_larden@xxxxxx




John Johnson
22/06/2006 14:14

To
Ian Larden/UK/IBM@xxxxxx
cc
vfr list
Subject
Re: VFR750F-L (1990) - lumpy running / backfire






On 21 Jun, 2006, at 18:10, Ian Larden wrote:

>
> Hi fellow listers - I have some questions for you all:
>
> My 1990 VFR is not running quite right - the symptoms are that on
> tickover
> it sounds 'lumpy', it backfires sometimes when I decelerate, and fuel
> consumption has decreased from around 45 to 35 mpg.
>
> In order to replace a rusted-through exhaust, I recently had to
> take off
> the front cylinder head, in order to remove a broken exhaust stud
> (it ended
> up getting drilled out by a local machine shop). When I replaced
> the head,
> I was not quite certain that I had got the valve timing gear teeth set
> correctly... although I followed the instructions in the Haynes
> workshop
> manual, it's not exactly clear where the timing marks need to be.
>
> So, my first question is, if I had re-assembled the valve timing
> gear, say,
> one or two teeth away from how it should be, would the engine even
> start /
> run at all?
>
I've no first-hand experience with these things, but the description
sounds about right for messed-up timing. fwiw, I _strongly_ recommend
owning the Honda shop manual.

Here's the instructions on replacing the cams from my Honda workshop
manual (90-96, USA printing) for comparison:

When only the front cylinder camshafts were removed:

Align the T1 mark on the starter clutch with the index mark on the
right crankcase cover.

Remove the rear cylinder head cover.

Make sure #1 cylinder is at TDC on the compression stroke. If it is
not, turn the crankshaft clockwise one full turn (360) and make sure
T1 aligns with the index mark on the right crankcase cover. Rotate
the crankshaft clockwise 1 1/4 turns (450) to align the T2 mark on
the starter clutch with the index mark on the right crankcase cover.

Apply molybdenum oil solution to the cam lobes.

Install the camshafts onto the front cylinder head so that the timing
_arrows_ on the camshaft gears are level with the cylinder head upper
edge and facing opposite each other. [picture points to the timing
arrows on the gears, and they are as far away from each other as they
can be.] NB: Honda appear to distinguish between timing _marks_ on
the rear camshafts and timing _arrows_ on the front.

INstall the camshaft holder dowel pins onto the rear [I'm hoping they
really mean front here] cylinder head. INstall the holders in the
correct positions, checking the identification marks on the holders
and the cylinder head. Tighten the holder bolts in the sequence as
shown. Torque 12N*m. Make sure the timing _arrows_ on the camshaft
gears are level with cylinder head upper edge and facing opposite
each other. [picture shows the cylinder head, with intake side on the
bottom, holder bolts are numbered (L-R, in four rows from top--
exhaust side-- to bottom--intake side--):4, 7, 6, 2; 8, 3, 1, 5; 5,
1, 3, 8; 2, 6, 7, 4;

Valve timing is correct when the timing _arrows_ are level with the
cylinder head upper edges, and the intake and exhaust timing _arrows_
face opposite each other.


> Secondly, if it's not the valve timing gear, what else might it be?
> Unbalanced carbs? Dirty/stale fuel? Anything else??
>
Unbalanced carbs can cause similar effects, but it typically doesn't
happen overnight. How suddenly did these symptoms come on?

later,
Johnj


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Vfr mailing list
Vfr@xxxxxx
For subscription and delivery options:
https://lists.cs.wisc.edu/mailman/listinfo/vfr

On 23 Jun, 2006, at 10:14, Ian Larden wrote:

>
> John, Ralph,
>
> Many thanks for your responses.
>
> To answer John's question - the symptoms started as soon as I got
> the engine running and took the bike out for a test run, which
> happened straight after fitting the new exhaust, which in turn
> necessitated removal & replacement of the front cylinder head, for
> reasons described in my previous post.
>
> I think the bit of the instructions missing from the Haynes manual is
> "Rotate the crankshaft clockwise 1 1/4 turns (450) to align the T2
> mark on the starter clutch with the index mark on the right
> crankcase cover" -
> I don't remember doing this, and I'm pretty sure I followed the
> steps in the Haynes manual.
>
> Anyway, I'm sufficiently convinced by your words to take the front
> valve timing gear off and re-position it.
>
> My follow-up question now is this - in which direction should I
> rotate the front timing gear - forwards or backwards one notch? Is
> it possible to deduce this from the symptoms I described?

I would _strongly_ suggest going through the entire procedure, as
described. I know that it's a lot of "extra" work, but it's going to
get things right the first time. IMO, the effort of yanking the valve
cover and cams even one more time is greater than the effort of
pulling both covers and doing it right once; as a result, I'm not
interested in risking having to do it twice. I'm not working on the
bike though, so it's up to you to evaluate your tolerance for that
sort of risk.

Of course, it might be obvious once you open up the front and spin
the engine through a full 4 strokes.

Whichever way you do it, good luck! :-)

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Vfr mailing list
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For subscription and delivery options:
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